d93b8109ae
The primary purpose of this change is to provide the ability to re-cache missing backing files on hard_reboot. The old pre_block_migration function was already performing a very similar operation. That function has been refactored to be idempotent and renamed to _create_images_and_backing. The pre_block_migration function is a wrapper, with some additional checking, around the renamed function. Image backend was also adjusted to look for either a missing backing file or disk image, recaching or creating accordingly. It should also be idempotent, never clobbering existing images. Change-Id: Icf4c488d6db59e732b463d08d0606b428ee1e7b9