vfio/igd: Check OpRegion support on GVT-g mdev

vfio/igd: Check OpRegion support on GVT-g mdev

The Intel GVT-g backend `kvmgt` always emulates OpRegion for vGPU,
make sure the OpRegion is present for enabling access to it
automatically later.

Also, hotplugging GVT-g vGPU is now always disallowed regardless of
OpRegion to prevent potential issues. Intel has never claimed support
for GVT-g hotplugging.

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/20250505170305.23622-6-tomitamoeko@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>