librbd: always open first parent image if it exists for a snapshot

librbd: always open first parent image if it exists for a snapshot

The deep-copy and migration features required force-opening the
parent image just in case the deep-flatten feature wasn't enabled
on an image. This change simplies the code by always opening the
direct parent image, which really only matters if a cloned image
has snapshots w/o the deep-flatten feature.

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