Prepend /dev/ to root_device_name in get_next_device_name

Because the API docs for booting an instance from a volume have shown to
use 'vda' for a long time, and it has worked, there may be instances
with root_device_name = 'vda'.  These instances will fail a check in
get_next_device_name when attempting to attach another volume, making
that impossible.  Another patch fixes new block_device_mappings by
prepending /dev/ when the root_device_name is populated, but this will
allow incorrectly populated instances to attach volumes.

Change-Id: I709a195c7e3f559315a2c307de62cbae8b72ac46
Partial-Bug: #1337821
Merge "Prepend /dev/ to root_device_name in get_next_device_name"