Prepend '/dev/' to supplied dev names in the API

Make sure that all user input device names end up in the database with
the '/dev/' prefix. This will make things more consistent and avoid
issues when attempting to parse device names.

This includes two main sources - block device mapping that can be passed
as part of a request or as image metadata, and root device name, passed
as image metadata.

Closes-bug: #1337821
Change-Id: Ibda82f511be99f1a68f2f77c72601a1b006be7a0
Merge "Prepend '/dev/' to supplied dev names in the API"