qemu-iotests: Test BLOCK_JOB_READY event for 0Kb image active commit

There should be a BLOCK_JOB_READY event with active commit, regardless
of image length. Let's test the 0 length image case, and make sure it
goes through the ready->complete process.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 8b9a30ca)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>