core: Graphics Device CRUD

This patch series is about modelling graphics device as regular engine
device.

This patch brings up GraphicsDevice that represents graphics framebuffer
(SPICE, VNC) and adds corresponding CRUD commands.

Change-Id: If9eed1ddb4aa8e8376ba5eff662f1bdf49fda800
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Bug-Url: https://bugzilla.redhat.com/1033547