vfio/igd: Refactor vfio_probe_igd_bar4_quirk into pci config quirk

vfio/igd: Refactor vfio_probe_igd_bar4_quirk into pci config quirk

The actual IO BAR4 write quirk in vfio_probe_igd_bar4_quirk was removed
in previous change, leaving the function not matching its name, so move
it into the newly introduced vfio_config_quirk_setup. There is no
functional change in this commit.

For now, to align with current legacy mode behavior, it returns and
proceeds on error. Later it will fail on error after decoupling the
quirks from legacy mode.

Signed-off-by: Tomita Moeko <tomitamoeko@gmail.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Tested-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Corvin Köhne <c.koehne@beckhoff.com>
Link: https://lore.kernel.org/qemu-devel/20250306180131.32970-7-tomitamoeko@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>