Recache or rebuild missing images on hard_reboot

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
Merge "Recache or rebuild missing images on hard_reboot"