Support for multiple heads for Qxl display device

When a user select Qxl as a video device he can channel multiple
monitors via a single PCI device instead of one PCI for each video
device.
This will save memory and PCI slots in the user guest OS.

Bug-Url: https://bugzilla.redhat.com/787578
Change-Id: If18b8ceba3d641f3e2e79947b34b549902f00b55
Signed-off-by: Shahar Havivi <shavivi@redhat.com>