Add a get_root_bdm utility function

Getting the root block device out of a list of block devices is a common
idiom and we use it in several places in our code base. This patch
factors it out in a handy utilty method and adds tests for it.

Change-Id: I4a2cb3c689fed9fbc0bc51ddba095e1269cacb2d
blueprint: clean-up-legacy-block-device-mapping
Merge "Add a get_root_bdm utility function"