vfio/igd: Allow overriding GMS with 0xf0 to 0xfe on Gen9+

vfio/igd: Allow overriding GMS with 0xf0 to 0xfe on Gen9+

On Gen9 and later IGD devices, GMS 0xf0 to 0xfe represents 4MB to 60MB
pre-allocated memory size in 4MB increments. Allow users overriding
GMS with these values.

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/20250505170305.23622-8-tomitamoeko@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>