vfio/igd: Require host VGA decode for legacy mode

vfio/igd: Require host VGA decode for legacy mode

Commit a59d06305fff ("vfio/pci: Introduce x-pci-class-code option")
allows user to expose non-VGA IGD device as VGA controller to the
guest. However, legacy mode requires host VGA range access. Check
that GGC.IVD == 0 before enabling legacy mode to ensure IGD is a real
VGA device claiming host VGA ranges.

Signed-off-by: Tomita Moeko <tomitamoeko@gmail.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250723160906.44941-2-tomitamoeko@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>