restapi:adjust api to new graphics representation

This patch changes REST API to internally use (new) graphics device
representation. Backwards compatibility is preserved (the interface for clients
is unchanged).

Change-Id: Ibed6bbbce8a36dff82a0903c3478cfb656589fb4
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Bug-Url: https://bugzilla.redhat.com/1033547