pci: Add INTx routing notifier

pci: Add INTx routing notifier

This per-device notifier shall be triggered by any interrupt router
along the path of a device's legacy interrupt signal on routing changes.
For simplicity reasons and as this is a slow path anyway, no further
details on the routing changes are provided. Instead, the callback is
expected to use pci_device_route_intx_to_irq to check the effect of the
change.

Will be used by KVM PCI device assignment and VFIO.

Signed-off-by: Jan Kiszka <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>