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
RBD driver will consider non-raw images to be uncloneable to trigger
automatic conversion to raw format.

Includes conversion of the corresponding unit test to use mock (instead
of mox) and expanded comments and error messages from patchset #58893 by
Edward Hope-Morley.

Change-Id: I5725d2f7576bc1b3e9b874ba944ad17d33a6e2cb
Closes-Bug: #1246219
Closes-Bug: #1247998