qemu-img: add check for zero-length job len

The mirror job doesn't update its total length until
it has already started running, so we should translate
a zero-length job-len as meaning 0%.

Otherwise, we may get divide-by-zero faults.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
