rbd-mirror: replace journal-specific parent image validation

rbd-mirror: replace journal-specific parent image validation

When cloning a mirrored image, instead of looking at the parent
image's journal state to tell if its still being synced, we can
instead verify that it doesn't have an rbd-mirror image sync
snapshots.

If it doesn't, the image isn't being actively synced, but it
might not have started either. However, the clone state machine
will open the local parent image and attempt to find the specified
snapshot. If it exists, the image sync is complete.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>