rbd: add CreateMirrorSnapshot implementing rbd_mirror_image_create_snapshot
rbd: add CreateMirrorSnapshot implementing rbd_mirror_image_create_snapshot

This function is used to "manually" trigger a snapshot for snapshot
based mirroring. It is needed as a pre-req to test actual cross-cluster
mirroring and the full functionality of GetGlobalMirrorStatus.

Signed-off-by: John Mulligan <eda8ad33169deab44f9fe65847f99e368da6c91b@redhat.com>

ceph/go-ceph,ceph/go-ceph,ceph/go-ceph