How timely, because I was in the process of trying to build Xen with that cpu hv_vendor_id spoofing patch.

I just tested with the nvidia-beta driver on a Arch Linux guest and from the kernel logs, the driver is loaded and sees the PCI device, but xorg crashes after loading the nvidia module complaining of "No devices detected".

This might be an xorg configuration thing, but it looks promising. I've already blown enough hours trying to get it to work today, so I'll try again in a few days.