vfio/igd: Detect IGD device by OpRegion

vfio/igd: Detect IGD device by OpRegion

There is currently no straightforward way to distinguish if a Intel
graphics device is IGD or discrete GPU. However, only IGD devices have
OpRegion. Use the presence of VFIO_REGION_SUBTYPE_INTEL_IGD_OPREGION
to identify IGD devices. Still, OpRegion on hotplugged IGD device is
not supported.

Signed-off-by: Tomita Moeko <tomitamoeko@gmail.com>
Reviewed-by: Corvin Köhne <c.koehne@beckhoff.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Tested-by: Alex Williamson <alex.williamson@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250505170305.23622-4-tomitamoeko@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>