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: I5725d2f7576bc1b3e9b874ba944ad17d33a6e2cb
Closes-Bug: #1246219