vfio/pci: Split vfio_intx_update()

This splits the vfio_intx_update() function into one part doing the actual
reconnection with the KVM irqchip (vfio_intx_update(), now taking an
argument with the new routing) and vfio_intx_routing_notifier() which
handles calls to the pci device intx routing notifier and calling
vfio_intx_update() when necessary.  This will make adding support for the
irqchip change notifier easier.

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>