Move libvirt RBD utilities to a new file

This will make it easier to share rbd-related code with
cinder and glance. Port the applicable unit tests over
from cinder.

Change-Id: I5bf08da87daf1d98f326f21cf33aba68aa830785
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Merge "Move libvirt RBD utilities to a new file"