rbd-mirror: update snapshot mirror image state after snapshot creation

rbd-mirror: update snapshot mirror image state after snapshot creation

The non-primary mirror snapshot is what is used to link the non-primary
to the primary image. If there is an interruption between creating the
non-primary image and the creation of the first non-primary snapshot,
the images will be considerered unlinked.

A future commit will modify librbd to avoid setting the mirror image
state to enabled for non-primary snapshot-based mirroring images.
rbd-mirror will already automatically delete images in the CREATING
state during the bootstrap phase.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit c9378ab8293e531a49970a01becd9762cf1f8f80)