spapr: handle pc-dimm unplug via hotplug handler chain

spapr: handle pc-dimm unplug via hotplug handler chain

Factor out memory unplug into separate function from spapr_lmb_release().
Then use generic hotplug_handler_unplug() to trigger memory unplug,
which will call spapr_machine_device_unplug() -> spapr_memory_unplug()
in the end.

This way unplug operation is not buried in lmb internals and located in
the same place like in other targets, following similar logic/call chain
across targets.

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