hw/isa/piix3: Wire up Xen PCI IRQ handling outside of PIIX3

hw/isa/piix3: Wire up Xen PCI IRQ handling outside of PIIX3

xen_intx_set_irq() doesn't depend on PIIX3State. In order to resolve
TYPE_PIIX3_XEN_DEVICE and in order to make Xen agnostic about the
precise south bridge being used, set up Xen's PCI IRQ handling of PIIX3
in the board.

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: <20230312120221.99183-4-shentey@gmail.com>
Message-Id: <20230403074124.3925-5-shentey@gmail.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>