core:Import/Export VM/Template with graphics dev

(backwards compatibility is preserved - when importing vm has no graphics
device and some video device, compatible graphics device is added to a vm on
import.)

Change-Id: Id14f0265f9f24e09be68b500b69b3eb5ac838413
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Bug-Url: https://bugzilla.redhat.com/1033547