frontend: RNG device final touches

This patch solves small bugs in RNG device frontend:
1: RNG device widgets are now nonchangable for vms in pools.
2: RNG sources unavailability messages were fixed.
3: In pool edit widget, RNG widgets weren't set according to backing
   entity.

Change-Id: I60a0c620f811d74aaef1efe8f570df1a4cc74857
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Bug-Url: https://bugzilla.redhat.com/977079