hw/isa/piix3: Avoid Xen-specific variant of piix3_write_config()

hw/isa/piix3: Avoid Xen-specific variant of piix3_write_config()

Subscribe to pci_bus_fire_intx_routing_notifier() instead which allows for
having a common piix3_write_config() for the PIIX3 device models.

While at it, move the subscription into machine code to facilitate resolving
TYPE_PIIX3_XEN_DEVICE.

In a possible future followup, pci_bus_fire_intx_routing_notifier() could
be adjusted in such a way that subscribing to it doesn't require
knowledge of the device firing it.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Tested-by: Chuck Zmudzinski <brchuckz@aol.com>
Message-Id: <20230312120221.99183-5-shentey@gmail.com>
Message-Id: <20230403074124.3925-6-shentey@gmail.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>