vfio/igd: Fix broken KVMGT OpRegion support

vfio/igd: Fix broken KVMGT OpRegion support

The KVMGT/GVT-g vGPU also exposes OpRegion. But unlike IGD passthrough,
it only needs the OpRegion quirk. A previous change moved x-igd-opregion
handling to config quirk breaks KVMGT functionality as it brings extra
checks and applied other quirks. Here we check if the device is mdev
(KVMGT) or not (passthrough), and then applies corresponding quirks.

As before, users must manually specify x-igd-opregion=on to enable it
on KVMGT devices. In the future, we may check the VID/DID and enable
OpRegion automatically.

Signed-off-by: Tomita Moeko <tomitamoeko@gmail.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Tested-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Corvin Köhne <c.koehne@beckhoff.com>
Link: https://lore.kernel.org/qemu-devel/20250306180131.32970-11-tomitamoeko@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>