hw/isa/vt82c686: Implement PCI IRQ routing

hw/isa/vt82c686: Implement PCI IRQ routing

The real VIA south bridges implement a PCI IRQ router which is configured
by the BIOS or the OS. In order to respect these configurations, QEMU
needs to implement it as well. The real chip may allow routing IRQs from
internal functions independently of PCI interrupts but since guests
usually configute it to a single shared interrupt we don't model that
here for simplicity.

Note: The implementation was taken from piix4_set_irq() in hw/isa/piix4.

Suggested-by: Bernhard Beschow <shentey@gmail.com>
Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Reviewed-by: Bernhard Beschow <shentey@gmail.com>
Tested-by: Rene Engel <ReneEngel80@emailn.de>
Message-Id: <fbb016c7d0e19093335c237e15f5f6c62c4393b4.1678188711.git.balaton@eik.bme.hu>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>