spapr: Handle irq backend changes with VFIO PCI devices

pseries machine type can have one of two different interrupt controllers in
use depending on feature negotiation with the guest.  Usually this is
invisible to devices, because they route to a common set of qemu_irqs which
in turn dispatch to the correct back end.

VFIO passthrough devices, however, wire themselves up directly to the KVM
irqchip for performance, which means they are affected by this change in
interrupt controller.  To get them to adjust correctly for the change in
irqchip, we need to fire the kvm irqchip change notifier.

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

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
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>