I've done some experimentation under qemu. On ACPI systems, Windows will 
*only* touch the keyboard controller if there's a device with an 
appropriate PNP HID or CID and if _STA evaluates to 0x0b or 0x0f. 
Otherwise it'll simply ignore the hardware entirely. By the looks of it 
their keyboard probing is also somewhat different to ours, but that's 
probably another story.

However, I did find a couple of device IDs that machines may produce 
which we don't currently check for. I'll send a patch.

-- 
Matthew Garrett | mjg59@srcf.ucam.org