Device assigned on KVM needs to know the mode
(enabled/inverted/disabled) and the IRQ number that a given device
triggers in the attached interrupt controller.

Add a PCI IRQ path discovery function that walks from a given device to
the host bridge, and gets this information.  For
this purpose, a host bridge callback function is introduced:
route_intx_to_irq. It is so far only implemented by the PIIX3, other
host bridges can be added later on as required.

Will be used for KVM PCI device assignment and VFIO.

Based on patch by Jan Kiszka, with minor tweaks.
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>