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.

Closes-Bug: #1346092
Co-Authored-By: Dmitry Borodaenko <dborodaenko@mirantis.com>
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Signed-off-by: Dmitry Borodaenko <dborodaenko@mirantis.com>
Change-Id: I614b8e2bf0ea9e71a73772cbabcf812b39fa8a34
Merge "Move libvirt RBD utilities to a new file"