[libvirt] libxl: Enable video device selection for Xen

Sorry, this fell complete off my todos for a while. So I split off the fixup of VRAM into a separate patch which may or may not be used and only accept vga, xen and cirrus as supported types in the main patch. I believe I saw some discussions about how to fix some of the VRAM values as they are passed into qemu. At least for the Cirrus type I saw that the command line looked ok but the guest was getting a much larger VRAM size than it was told. -Stefan