vm: graphdev: update the _devices field

this patch let the Graphic Devices instances in _devices
be updated from the fresh data from libvirt.

Change-Id: Ie6a2133742a5515dc1077cd935ad7d1e37410e44
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27932
Reviewed-by: Dan Kenigsberg <danken@redhat.com>