Create an image BDM for every instance

This patch makes every boot create a block device mapping if the
instance was started with an image.

These block devices are not used anywhere at the moment but will be used
in the future as part of a more flexible block device API.

This patch also makes sure they are deleted once the instance is deleted
as well.

Change-Id: Ia89d531be71c460f1f82fcfce34b270639a23061
blueprint: improve-block-device-handling
API extension for the new BDM format

Change-Id: I2c1b63e41deca26f727fb9ed912a55494db9c76c
Add basic API BDM validations

Change-Id: Ie4c85aa16ada2ce7b6873d46cc70538e316e18be