hw/i386/pc_piix: Make piix_intx_routing_notifier_xen() more device independent

hw/i386/pc_piix: Make piix_intx_routing_notifier_xen() more device independent

This is a follow-up on commit 89965db43cce "hw/isa/piix3: Avoid Xen-specific
variant of piix3_write_config()" which introduced
piix_intx_routing_notifier_xen(). This function is implemented in board code but
accesses the PCI configuration space of the PIIX ISA function to determine the
PCI interrupt routes. Avoid this by reusing pci_device_route_intx_to_irq() which
makes piix_intx_routing_notifier_xen() more device-agnostic.

One remaining improvement would be making piix_intx_routing_notifier_xen()
agnostic towards the number of PCI interrupt routes and move it to xen-hvm.
This might be useful for possible Q35 Xen efforts but remains a future exercise
for now.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240107231623.5282-1-shentey@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>