[libvirt] libxl: Enable video device selection for Xen

[Stefan Bader]

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