librbd: avoid refreshing within the mirror state machines

librbd: avoid refreshing within the mirror state machines

The refresh has been moved up to the API layer. Also avoid querying
the MirrorImage struct when creating mirror primary snapshots since
(1) when enabling the record won't exist yet and (2) in the demote/
promote cases it would have just been read by the previous state
machine.

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