discover: Consolidate device path, name and ID.

This change cleans up our usage of device path, names and IDs.

Device ID is the kernel name for the device. We also expose this through
lookup_by_name.

Device path is the path to the dev node (ie, always starts with /dev/),
and is only used for mounting.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>