core: prevent running vms when rng is not supported

This patch prevents vms with active rng device to run when rng device is
not required by cluster. This situation could possibly happen when admin
deselects required rng sources on cluster level, but there are already
vms with rng (with such source) created in the cluster.

Change-Id: Ia67a0a9b2a000670d60e3cc362c93023ddf69f38
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>