mirror: Go through ready -> complete process for 0 len image

When mirroring or active committing a zero length image, BLOCK_JOB_READY
is not reported now, instead the job completes because we short circuit
the mirror job loop.

This is inconsistent with non-zero length images, and only confuses
management software.

Let's do the same thing when seeing a 0-length image: report ready
immediately; wait for block-job-cancel or block-job-complete; clear the
cancel flag as existing non-zero image synced case (cancelled after
ready); then jump to the exit.

Reported-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>