core: RNG fixes

1, Reset rng sources of default cluster to NO sources (centos/rhel hosts
   don't support rng passthrough).
2, Adjust user filtering of GetRngDeviceQuery (now it should work in
   both Webadmin and Userportal).

Change-Id: I0bf6715daba3f1f7121d636fef3fa1240301f452
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Bug-Url: https://bugzilla.redhat.com/1123385