[PATCH v2 05/17] iotests/040: Fix TestCommitWithFilters test

[John Snow <jsnow@redhat.com>]

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
  ┗━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

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

[Patch entirely written by Hanna but I don't have her S-o-B]
[My commit message is probably also garbage, sorry]
[Feel free to suggest a better one]
[I hope your day is going well]
Signed-off-by: John Snow <jsnow@redhat.com>

Signed-off-by: John Snow <jsnow@redhat.com>
---
 tests/qemu-iotests/040 | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/tests/qemu-iotests/040 b/tests/qemu-iotests/040
index c4a90937dc..30eb97829e 100755
--- a/tests/qemu-iotests/040
+++ b/tests/qemu-iotests/040
@@ -836,7 +836,8 @@ class TestCommitWithFilters(iotests.QMPTestCase):
                              job_id='commit',
                              device='top-filter',
                              top_node='cow-2',
-                             base_node='cow-1')
+                             base_node='cow-1',
+                             backing_file=self.img1)
         self.assert_qmp(result, 'return', {})
         self.wait_until_completed(drive='commit')
 
@@ -852,7 +853,8 @@ class TestCommitWithFilters(iotests.QMPTestCase):
                              job_id='commit',
                              device='top-filter',
                              top_node='cow-1',
-                             base_node='cow-0')
+                             base_node='cow-0',
+                             backing_file=self.img0)
         self.assert_qmp(result, 'return', {})
         self.wait_until_completed(drive='commit')
 
-- 
2.34.1