backend: Control virtio rng device

This patch adds the possibility of attaching/detaching virtio
paravirualized device for VMs.

This adds a new object VmRngDevice that represents the device and
updates related command & parameter classes.

Change-Id: Ifc26a484c84244074426f4b0c790e2cc4ebb8da6
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Bug-Url: https://bugzilla.redhat.com/977079