Update probe code for backend layout.

Update probe code for backend layout.
Backend directories are of form backend/<type>/<frontend-uuid>/<id>.
This extra level makes an asymmetry with front ends (device/type/id) and
are too long to make nice bus-ids under Linux, but the code isn't too
horrible, and an OS which only wants to be a guest doesn't need to implement
it. It allows backends to calculate the front-end path directly, which
is a nice benefit.
Also: subtype matching is tightened: they must match exactly. That way a future
variant which requires a specific driver will not match current drivers.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Christian Limpach <Christian.Limpach@cl.cam.ac.uk>