vfio/igd: Restrict legacy mode to Gen6-9 devices

vfio/igd: Restrict legacy mode to Gen6-9 devices

Intel only provides legacy VBIOS for IGD up to Gen9, and there is no
CSM support on later devices. Additionally, Seabios can only handle
32-bit BDSM register used until Gen9. Since legacy mode requires VGA
capability, restrict it to Gen6 through Gen9 devices.

Link: https://lore.kernel.org/qemu-devel/20250325172239.27926-1-tomitamoeko@gmail.com/T/
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/20250505170305.23622-2-tomitamoeko@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>