frontend: Control virtio rng device

This patch adds the possibility of attaching/detaching virtio
paravirualized device for VMs. For this a new subtab is added to
new/edit vm dialog.

Change-Id: Iffbc48062f25d90795bc360362f42cd1042fb888
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Bug-Url: https://bugzilla.redhat.com/977079