vfio/pci: Respond to KVM irqchip change notifier

VFIO PCI devices already respond to the pci intx routing notifier, in order
to update kernel irqchip mappings when routing is updated.  However this
won't handle the case where the irqchip itself is replaced by a different
model while retaining the same routing.  This case can happen on
the pseries machine type due to PAPR feature negotiation.

To handle that case, add a handler for the irqchip change notifier, which
does much the same thing as the routing notifier, but is unconditional,
rather than being a no-op when the routing hasn't changed.

Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Alexey Kardashevskiy <aik@ozlabs.ru>

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Tested-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Acked-by: Alex Williamson <alex.williamson@redhat.com>