hw/isa/piix3: Reuse piix3_realize() in piix3_xen_realize()

hw/isa/piix3: Reuse piix3_realize() in piix3_xen_realize()

This is a preparational patch for the next one to make the following
more obvious:

First, pci_bus_irqs() is now called twice in case of Xen where the
second call overrides the pci_set_irq_fn with the Xen variant.

Second, pci_bus_set_route_irq_fn() is now also called in Xen mode.

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: <20230312120221.99183-3-shentey@gmail.com>
Message-Id: <20230403074124.3925-4-shentey@gmail.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>