virt-libvirt: Impl. libvirt_xml graphics device

* Simple/minimal implementation with passwd property, more can be added
  later.
* Thanks to Kyla - Wei Zhang <weizhan@redhat.com> for requesting and
  helping test.
* Added unittest for graphics device to verify changes are commited to
  vm_xml instance.

Signed-off-by: Chris Evich <cevich@redhat.com>