core: add graphics device to management classes

This patch adds graphics device to vm/template management (CRUD)
commands.

Change-Id: I39c84ce94cd3cb52286c52d765a389e69c9b0e62
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Bug-Url: https://bugzilla.redhat.com/1033547