vfio/igd: Fix VGA regions are not exposed in legacy mode

vfio/igd: Fix VGA regions are not exposed in legacy mode

In commit a59d06305fff ("vfio/pci: Introduce x-pci-class-code option"),
pci_register_vga() has been moved ouside of vfio_populate_vga(). As a
result, IGD VGA ranges are no longer properly exposed to guest.

To fix this, call pci_register_vga() after vfio_populate_vga() legacy
mode. A wrapper function vfio_pci_config_register_vga() is introduced
to handle it.

Fixes: a59d06305fff ("vfio/pci: Introduce x-pci-class-code option")
Signed-off-by: Tomita Moeko <tomitamoeko@gmail.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250723160906.44941-3-tomitamoeko@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>