engine: add support for ram/vram on QXL device

setting support for qxl device with proper ram/vram attributes:

1 Monitor, Single PCI checked     64MB   32MB
2 Monitors, Single PCI checked    128MB  32MB
4 Monitors, Single PCI checked    256MB  32MB

Change-Id: I0d380901daea93c3d6dbdfee48addf190e15ed5a
Bug-Url: https://bugzilla.redhat.com/787578
Signed-off-by: Shahar Havivi <shaharh@redhat.com>
Signed-off-by: Omer Frenkel <ofrenkel@redhat.com>