virt: move rng device to vmdevices.py

No real code changes

Change-Id: Ide65e4125f43a9bf8289ec1814086d1875f5eec5
Signed-off-by: Martin Polednik <mpolednik@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/34305
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>