DRYing up volume_in_mapping code.

Libvirt, VMWare, and Xen drivers all reimplement this method. This
refactors the code into the `block_device` module so all drivers can
share it.

Future Work: Refactor xen so that strip_prefix doesn't need to be passed
to `volume_in_mapping`.

Change-Id: I026b655f2eb7ea9cebe84058749c05ce93bcb7d4
Merge "DRYing up volume_in_mapping code."