iotests/040: Fix TestCommitWithFilters test

iotests/040: Fix TestCommitWithFilters test

Without this change, asserting that qemu_io always returns 0 causes this
test to fail in a way we happened not to be catching previously:

 qemu.utils.VerboseProcessError: Command
  '('/home/jsnow/src/qemu/bin/git/tests/qemu-iotests/../../qemu-io',
  '--cache', 'writeback', '--aio', 'threads', '-f', 'qcow2', '-c',
  'read -P 4 3M 1M',
  '/home/jsnow/src/qemu/bin/git/tests/qemu-iotests/scratch/3.img')'
  returned non-zero exit status 1.
  ┏━ output ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
  ┃ qemu-io: can't open device
  ┃ /home/jsnow/src/qemu/bin/git/tests/qemu-iotests/scratch/3.img:
  ┃ Could not open backing file: Could not open backing file: Throttle
  ┃ group 'tg' does not exist
  ┗━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

The commit jobs changes the backing file string stored in the image file
header belonging to the node above the commit’s top node to point to the
commit target (the base node).  QEMU tries to be as accurate as
possible, and so in these test cases will include the filter that is
part of the block graph in that backing file string (by virtue of making
it a json:{} description of the post-commit subgraph).  This makes
little sense outside of QEMU, though: Specifically, the throttle node in
that subgraph will dearly miss its supposedly associated throttle group
object.

When starting the commit job, we can specify a custom backing file
string to write into said image file, so let’s use that feature to write
the plain filename of the backing chain’s next actual image file there.

Explicitly provide the backing file so that opening the file outside of
QEMU (Where we will not have throttle groups) will succeed.

Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20220418211504.943969-6-jsnow@redhat.com>