Use snapshot metadata usage to determine if snapshot is empty

Use snapshot metadata usage to determine if snapshot is empty

Version >= 1.8.0 of the DM snapshot target appends metadata sectors used
to a snapshot's status.  This patch allows LVM2 to accurately determine
if the snapshot store is empty.  Knowing when a snapshot store is empty
is important in the context of snapshot-merge (means merge is complete).

Also update LVM2 to be aware of the possibility for "Merge failed" in
the snapshot-merge target's status.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>