python: use vm.cmd() instead of vm.qmp() where appropriate

In many cases we just want an effect of qmp command and want to raise
on failure. Use vm.cmd() method which does exactly this.

The commit

python: use vm.cmd() instead of vm.qmp() where appropriate

In many cases we just want an effect of qmp command and want to raise
on failure. Use vm.cmd() method which does exactly this.

The commit is generated by command

git grep -l '\.qmp(' | xargs ./scripts/python_qmp_updater.py

And then, fix self.assertRaises to expect ExecuteError exception in
tests/qemu-iotests/124

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20231006154125.1068348-16-vsementsov@yandex-team.ru
Signed-off-by: John Snow <jsnow@redhat.com>

show more ...

python: use vm.cmd() instead of vm.qmp() where appropriate

In many cases we just want an effect of qmp command and want to raise
on failure. Use vm.cmd() method which does exactly this.

The commit

python: use vm.cmd() instead of vm.qmp() where appropriate

In many cases we just want an effect of qmp command and want to raise
on failure. Use vm.cmd() method which does exactly this.

The commit is generated by command

git grep -l '\.qmp(' | xargs ./scripts/python_qmp_updater.py

And then, fix self.assertRaises to expect ExecuteError exception in
tests/qemu-iotests/124

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20231006154125.1068348-16-vsementsov@yandex-team.ru
Signed-off-by: John Snow <jsnow@redhat.com>

show more ...

iotests/245: Check if 'compress' driver is available

Skip TestBlockdevReopen.test_insert_compress_filter() if the 'compress'
driver isn't available.

In order to make the test succeed when the case

iotests/245: Check if 'compress' driver is available

Skip TestBlockdevReopen.test_insert_compress_filter() if the 'compress'
driver isn't available.

In order to make the test succeed when the case is skipped, we also need
to remove any output from it (which would be missing in the case where
we skip it). This is done by replacing qemu_io_log() with qemu_io(). In
case of failure, qemu_io() raises an exception with the output of the
qemu-io binary in its message, so we don't actually lose anything.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230511143801.255021-1-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

show more ...

iotests/245: Check if 'compress' driver is available

Skip TestBlockdevReopen.test_insert_compress_filter() if the 'compress'
driver isn't available.

In order to make the test succeed when the case

iotests/245: Check if 'compress' driver is available

Skip TestBlockdevReopen.test_insert_compress_filter() if the 'compress'
driver isn't available.

In order to make the test succeed when the case is skipped, we also need
to remove any output from it (which would be missing in the case where
we skip it). This is done by replacing qemu_io_log() with qemu_io(). In
case of failure, qemu_io() raises an exception with the output of the
qemu-io binary in its message, so we don't actually lose anything.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230511143801.255021-1-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

show more ...

iotests: rebase qemu_io() on top of qemu_tool()

Rework qemu_io() to be analogous to qemu_img(); a function that requires
a return code of zero by default unless disabled explicitly.

Tests that use

iotests: rebase qemu_io() on top of qemu_tool()

Rework qemu_io() to be analogous to qemu_img(); a function that requires
a return code of zero by default unless disabled explicitly.

Tests that use qemu_io():
030 040 041 044 055 056 093 124 129 132 136 148 149 151 152 163 165 205
209 219 236 245 248 254 255 257 260 264 280 298 300 302 304
image-fleecing migrate-bitmaps-postcopy-test migrate-bitmaps-test
migrate-during-backup migration-permissions

Test that use qemu_io_log():
242 245 255 274 303 307 nbd-reconnect-on-open

Copy-pastables for testing/verification:

./check -qcow2 030 040 041 044 055 056 124 129 132 151 152 163 165 209 \
219 236 242 245 248 254 255 257 260 264 274 \
280 298 300 302 303 304 307 image-fleecing \
migrate-bitmaps-postcopy-test migrate-bitmaps-test \
migrate-during-backup nbd-reconnect-on-open
./check -raw 093 136 148 migration-permissions
./check -nbd 205

# ./configure configure --disable-gnutls --enable-gcrypt
# this ALSO requires passwordless sudo.
./check -luks 149

# Just the tests that were edited in this commit:
./check -qcow2 030 040 242 245
./check -raw migration-permissions
./check -nbd 205
./check -luks 149

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20220418211504.943969-8-jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>

show more ...

iotests: Test blockdev-reopen with iothreads and throttling

The 'throttle' block driver implements .bdrv_co_drain_end, so
blockdev-reopen will have to wait for it to complete in the polling
loop at

iotests: Test blockdev-reopen with iothreads and throttling

The 'throttle' block driver implements .bdrv_co_drain_end, so
blockdev-reopen will have to wait for it to complete in the polling
loop at the end of qmp_blockdev_reopen(). This makes AIO_WAIT_WHILE()
release the AioContext lock, which causes a crash if the lock hasn't
correctly been taken.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20220203140534.36522-3-kwolf@redhat.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

show more ...

qapi: Improve input_type_enum()'s error message

The error message claims the parameter is invalid:

$ qemu-system-x86_64 -object qom-type=nonexistent
qemu-system-x86_64: -object qom-type=non

qapi: Improve input_type_enum()'s error message

The error message claims the parameter is invalid:

$ qemu-system-x86_64 -object qom-type=nonexistent
qemu-system-x86_64: -object qom-type=nonexistent: Invalid parameter 'nonexistent'

What's wrong is actually the *value* 'nonexistent'. Improve the
message to

qemu-system-x86_64: -object qom-type=nonexistent: Parameter 'qom-type' does not accept value 'nonexistent'

Fixes: https://gitlab.com/qemu-project/qemu/-/issues/608
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20211020180231.434071-1-armbru@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>

show more ...

iotests/245: Fix type for iothread property

iothread is a string property, so None (= JSON null) is not a valid
value for it. Pass the empty string instead to get the default iothread.

Signed-off-b

iotests/245: Fix type for iothread property

iothread is a string property, so None (= JSON null) is not a valid
value for it. Pass the empty string instead to get the default iothread.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20211008133442.141332-6-kwolf@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

show more ...

block: Make blockdev-reopen stable API

This patch drops the 'x-' prefix from x-blockdev-reopen.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewe

block: Make blockdev-reopen stable API

This patch drops the 'x-' prefix from x-blockdev-reopen.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210708114709.206487-7-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

show more ...

iotests: Test reopening multiple devices at the same time

This test swaps the images used by two active block devices.

This is now possible thanks to the new ability to run
x-blockdev-reopen on mul

iotests: Test reopening multiple devices at the same time

This test swaps the images used by two active block devices.

This is now possible thanks to the new ability to run
x-blockdev-reopen on multiple devices at the same time.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210708114709.206487-6-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

show more ...

block: Support multiple reopening with x-blockdev-reopen

[ kwolf: Fixed AioContext locking ]

Signed-off-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-b

block: Support multiple reopening with x-blockdev-reopen

[ kwolf: Fixed AioContext locking ]

Signed-off-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210708114709.206487-5-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

show more ...

iotests: Test replacing files with x-blockdev-reopen

This patch adds new tests in which we use x-blockdev-reopen to change
bs->file

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-Id: <202

iotests: Test replacing files with x-blockdev-reopen

This patch adds new tests in which we use x-blockdev-reopen to change
bs->file

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-Id: <20210610120537.196183-10-vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

show more ...

block: Allow changing bs->file on reopen

When the x-blockdev-reopen was added it allowed reconfiguring the
graph by replacing backing files, but changing the 'file' option was
forbidden. Because of

block: Allow changing bs->file on reopen

When the x-blockdev-reopen was added it allowed reconfiguring the
graph by replacing backing files, but changing the 'file' option was
forbidden. Because of this restriction some operations are not
possible, notably inserting and removing block filters.

This patch adds support for replacing the 'file' option. This is
similar to replacing the backing file and the user is likewise
responsible for the correctness of the resulting graph, otherwise this
can lead to data corruption.

Signed-off-by: Alberto Garcia <berto@igalia.com>
[vsementsov: bdrv_reopen_parse_file_or_backing() is modified a lot]
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210610120537.196183-9-vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

show more ...

block: bdrv_reopen_parse_backing(): don't check frozen child

bdrv_set_backing_noperm() takes care of it (actual check is in
bdrv_set_file_or_backing_noperm()), so we don't need to check it here.

Wh

block: bdrv_reopen_parse_backing(): don't check frozen child

bdrv_set_backing_noperm() takes care of it (actual check is in
bdrv_set_file_or_backing_noperm()), so we don't need to check it here.

While being here, improve error message a bit.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210610120537.196183-5-vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

show more ...

block: refactor bdrv_node_check_perm()

Now, bdrv_node_check_perm() is called only with fresh cumulative
permissions, so its actually "refresh_perm".

Move permission calculation to the function. Als

block: refactor bdrv_node_check_perm()

Now, bdrv_node_check_perm() is called only with fresh cumulative
permissions, so its actually "refresh_perm".

Move permission calculation to the function. Also, drop unreachable
error message and rewrite the remaining one to be more generic (as now
we don't know which node is added and which was already here).

Add also Virtuozzo copyright, as big work is done at this point.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20210428151804.439460-37-vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

show more ...

blockdev: Clarify error messages pertaining to 'node-name'

Signed-off-by: Connor Kuehl <ckuehl@redhat.com>
Message-Id: <20210305151929.1947331-3-ckuehl@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@r

blockdev: Clarify error messages pertaining to 'node-name'

Signed-off-by: Connor Kuehl <ckuehl@redhat.com>
Message-Id: <20210305151929.1947331-3-ckuehl@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

show more ...

block: Clarify error messages pertaining to 'node-name'

Some error messages contain ambiguous representations of the 'node-name'
parameter. This can be particularly confusing when exchanging QMP
mes

block: Clarify error messages pertaining to 'node-name'

Some error messages contain ambiguous representations of the 'node-name'
parameter. This can be particularly confusing when exchanging QMP
messages (C = client, S = server):

C: {"execute": "block_resize", "arguments": { "device": "my_file", "size": 26843545600 }}
S: {"error": {"class": "GenericError", "desc": "Cannot find device=my_file nor node_name="}}
^^^^^^^^^

This error message suggests one could send a message with a key called
'node_name':

C: {"execute": "block_resize", "arguments": { "node_name": "my_file", "size": 26843545600 }}
^^^^^^^^^

but using the underscore is actually incorrect, the parameter should be
'node-name':

S: {"error": {"class": "GenericError", "desc": "Parameter 'node_name' is unexpected"}}

This behavior was uncovered in bz1651437, but I ended up going down a
rabbit hole looking for other areas where this miscommunication might
occur and changing those accordingly as well.

Fixes: https://bugzilla.redhat.com/1651437
Signed-off-by: Connor Kuehl <ckuehl@redhat.com>
Message-Id: <20210305151929.1947331-2-ckuehl@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

show more ...

iotests: Drop deprecated 'props' from object-add

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-Id: <20210222115737.2993-1-berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block: apply COR-filter to block-stream jobs

This patch completes the series with the COR-filter applied to
block-stream operations.

Adding the filter makes it possible in future implement discardi

block: apply COR-filter to block-stream jobs

This patch completes the series with the COR-filter applied to
block-stream operations.

Adding the filter makes it possible in future implement discarding
copied regions in backing files during the block-stream job, to reduce
the disk overuse (we need control on permissions).

Also, the filter now is smart enough to do copy-on-read with specified
base, so we have benefit on guest reads even when doing block-stream of
the part of the backing chain.

Several iotests are slightly modified due to filter insertion.

Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20201216061703.70908-14-vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

show more ...

iotests: define group in each iotest

We are going to drop group file. Define group in tests as a preparatory
step.

The patch is generated by

cd tests/qemu-iotests

grep '^[0-9]\{3\} ' grou

iotests: define group in each iotest

We are going to drop group file. Define group in tests as a preparatory
step.

The patch is generated by

cd tests/qemu-iotests

grep '^[0-9]\{3\} ' group | while read line; do
file=$(awk '{print $1}' <<< "$line");
groups=$(sed -e 's/^... //' <<< "$line");
awk "NR==2{print \"# group: $groups\"}1" $file > tmp;
cat tmp > $file;
done

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210116134424.82867-7-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>

show more ...

block: Leave BDS.backing_{file,format} constant

Parts of the block layer treat BDS.backing_file as if it were whatever
the image header says (i.e., if it is a relative path, it is relative to
the ov

block: Leave BDS.backing_{file,format} constant

Parts of the block layer treat BDS.backing_file as if it were whatever
the image header says (i.e., if it is a relative path, it is relative to
the overlay), other parts treat it like a cache for
bs->backing->bs->filename (relative paths are relative to the CWD).
Considering bs->backing->bs->filename exists, let us make it mean the
former.

Among other things, this now allows the user to specify a base when
using qemu-img to commit an image file in a directory that is not the
CWD (assuming, everything uses relative filenames).

Before this patch:

$ ./qemu-img create -f qcow2 foo/bot.qcow2 1M
$ ./qemu-img create -f qcow2 -b bot.qcow2 foo/mid.qcow2
$ ./qemu-img create -f qcow2 -b mid.qcow2 foo/top.qcow2
$ ./qemu-img commit -b mid.qcow2 foo/top.qcow2
qemu-img: Did not find 'mid.qcow2' in the backing chain of 'foo/top.qcow2'
$ ./qemu-img commit -b foo/mid.qcow2 foo/top.qcow2
qemu-img: Did not find 'foo/mid.qcow2' in the backing chain of 'foo/top.qcow2'
$ ./qemu-img commit -b $PWD/foo/mid.qcow2 foo/top.qcow2
qemu-img: Did not find '[...]/foo/mid.qcow2' in the backing chain of 'foo/top.qcow2'

After this patch:

$ ./qemu-img commit -b mid.qcow2 foo/top.qcow2
Image committed.
$ ./qemu-img commit -b foo/mid.qcow2 foo/top.qcow2
qemu-img: Did not find 'foo/mid.qcow2' in the backing chain of 'foo/top.qcow2'
$ ./qemu-img commit -b $PWD/foo/mid.qcow2 foo/top.qcow2
Image committed.

With this change, bdrv_find_backing_image() must look at whether the
user has overridden a BDS's backing file. If so, it can no longer use
bs->backing_file, but must instead compare the given filename against
the backing node's filename directly.

Note that this changes the QAPI output for a node's backing_file. We
had very inconsistent output there (sometimes what the image header
said, sometimes the actual filename of the backing image). This
inconsistent output was effectively useless, so we have to decide one
way or the other. Considering that bs->backing_file usually at runtime
contained the path to the image relative to qemu's CWD (or absolute),
this patch changes QAPI's backing_file to always report the
bs->backing->bs->filename from now on. If you want to receive the image
header information, you have to refer to full-backing-filename.

This necessitates a change to iotest 228. The interesting information
it really wanted is the image header, and it can get that now, but it
has to use full-backing-filename instead of backing_file. Because of
this patch's changes to bs->backing_file's behavior, we also need some
reference output changes.

Along with the changes to bs->backing_file, stop updating
BDS.backing_format in bdrv_backing_attach() as well. This way,
ImageInfo's backing-filename and backing-filename-format fields will
represent what the image header says and nothing else.

iotest 245 changes in behavior: With the backing node no longer
overriding the parent node's backing_file string, you can now omit the
@backing option when reopening a node with neither a default nor a
current backing file even if it used to have a backing node at some
point.

273 also changes: The base image is opened without a format layer, so
ImageInfo.backing-filename-format used to report "file" for the base
image's overlay after blockdev-snapshot. However, the image header
never says "file" anywhere, so it now reports $IMGFMT.

Signed-off-by: Max Reitz <mreitz@redhat.com>

show more ...

iotests: Specify explicit backing format where sensible

There are many existing qcow2 images that specify a backing file but
no format. This has been the source of CVEs in the past, but has
become

iotests: Specify explicit backing format where sensible

There are many existing qcow2 images that specify a backing file but
no format. This has been the source of CVEs in the past, but has
become more prominent of a problem now that libvirt has switched to
-blockdev. With older -drive, at least the probing was always done by
qemu (so the only risk of a changed format between successive boots of
a guest was if qemu was upgraded and probed differently). But with
newer -blockdev, libvirt must specify a format; if libvirt guesses raw
where the image was formatted, this results in data corruption visible
to the guest; conversely, if libvirt guesses qcow2 where qemu was
using raw, this can result in potential security holes, so modern
libvirt instead refuses to use images without explicit backing format.

The change in libvirt to reject images without explicit backing format
has pointed out that a number of tools have been far too reliant on
probing in the past. It's time to set a better example in our own
iotests of properly setting this parameter.

iotest calls to create, rebase, and convert are all impacted to some
degree. It's a bit annoying that we are inconsistent on command line
- while all of those accept -o backing_file=...,backing_fmt=..., the
shortcuts are different: create and rebase have -b and -F, while
convert has -B but no -F. (amend has no shortcuts, but the previous
patch just deprecated the use of amend to change backing chains).

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200706203954.341758-9-eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

show more ...

iotests: use python logging for iotests.log()

We can turn logging on/off globally instead of per-function.

Remove use_log from run_job, and use python logging to turn on
diffable output when we run

iotests: use python logging for iotests.log()

We can turn logging on/off globally instead of per-function.

Remove use_log from run_job, and use python logging to turn on
diffable output when we run through a script entry point.

iotest 245 changes output order due to buffering reasons.

An extended note on python logging:

A NullHandler is added to `qemu.iotests` to stop output from being
generated if this code is used as a library without configuring logging.
A NullHandler is only needed at the root, so a duplicate handler is not
needed for `qemu.iotests.diff_io`.

When logging is not configured, messages at the 'WARNING' levels or
above are printed with default settings. The NullHandler stops this from
occurring, which is considered good hygiene for code used as a library.

See https://docs.python.org/3/howto/logging.html#library-config

When logging is actually enabled (always at the behest of an explicit
call by a client script), a root logger is implicitly created at the
root, which allows messages to propagate upwards and be handled/emitted
from the root logger with default settings.

When we want iotest logging, we attach a handler to the
qemu.iotests.diff_io logger and disable propagation to avoid possible
double-printing.

For more information on python logging infrastructure, I highly
recommend downloading the pip package `logging_tree`, which provides
convenient visualizations of the hierarchical logging configuration
under different circumstances.

See https://pypi.org/project/logging_tree/ for more information.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200331000014.11581-15-jsnow@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

show more ...

block: bdrv_reopen() with backing file in different AioContext

This patch allows bdrv_reopen() (and therefore the x-blockdev-reopen QMP
command) to attach a node as the new backing file even if the

block: bdrv_reopen() with backing file in different AioContext

This patch allows bdrv_reopen() (and therefore the x-blockdev-reopen QMP
command) to attach a node as the new backing file even if the node is in
a different AioContext than the parent if one of both nodes can be moved
to the AioContext of the other node.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Tested-by: Peter Krempa <pkrempa@redhat.com>
Message-Id: <20200306141413.30705-3-kwolf@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

show more ...

iotests: Refactor blockdev-reopen test for iothreads

We'll want to test more than one successful case in the future, so
prepare the test for that by a refactoring that runs each scenario in a
separa

iotests: Refactor blockdev-reopen test for iothreads

We'll want to test more than one successful case in the future, so
prepare the test for that by a refactoring that runs each scenario in a
separate VM.

test_iothreads_switch_{backing,overlay} currently produce errors, but
these are cases that should actually work, by switching either the
backing file node or the overlay node to the AioContext of the other
node.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Tested-by: Peter Krempa <pkrempa@redhat.com>
Message-Id: <20200306141413.30705-2-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

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