core: engine -> vdsm part for spicevnc

this patch (and its follow-up) makes graphics device "alive" - now
graphics device is used for internal representation of state of graphics
stuff.

Change-Id: Ie6eef54aea60e8fcd7f0d255c6cf72205ae45838
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Bug-Url: https://bugzilla.redhat.com/1033547