Do not clone non-raw images in rbd backend

RBD backend only supports booting from images in raw format. A volume
that was cloned from an image in any other format is not bootable. The
driver should consider non-raw images to be uncloneable to trigger
automatic conversion to raw format.

Change-Id: Ie82fb825814f304355b5ce0d6621d07dcbd75c7c
Closes-Bug: #1246219