spapr_pci: perform unplug via the hotplug handler

Introduce and use the "unplug" callback.

This is a preparation for multi-stage hotplug handlers, whereby the bus
hotplug handler is overwritten by the machine hotplug handler. This handler
will then pass control to the bus hotplug handler. So to get this running
cleanly, we also have to make sure to go via the hotplug handler chain when
actually unplugging a device after an unplug request. Lookup the hotplug
handler and call "unplug".

Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>