frontend: RNG device sources reporting

This is a follow-up to original VirtIO RNG patches (18176, 18497).

This patch adds
 - required rng sources to cluster (set by admin),
 - supported rng sources to host (reported by VDSM).

The host-supported rng sources must comply with cluster-required rng
sources, otherwise corresponding host is set to non-operational state.

Frontend changes:
- added checkboxes for required rng sources on Cluster view (checkboxes
  for 'random' and 'hwrng' sources are added)

Change-Id: Ifd065835d18575b686ee805b032662205b31c966
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Bug-Url: https://bugzilla.redhat.com/977079