iotests: Fix 141 when run with qed

iotests: Fix 141 when run with qed

69f47505ee has changed qcow2 in such a way that the commit job run in
test 141 (and 144[1]) returns before it emits the READY event.  However,
141 also runs with qed, where the order is still the other way around.
Just filter out the {"return": {}} so the test passes for qed again.

[1] 144 only runs with qcow2, so it is fine as it is.

Suggested-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Fixes: 69f47505ee66afaa513305de0c1895a224e52c45
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190809185253.17535-1-mreitz@redhat.com
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>