spapr: make cpu core unplug follow expected hotunplug call flow

spapr: make cpu core unplug follow expected hotunplug call flow

spapr_core_unplug() were essentially spapr_core_unplug_request()
handler that requested CPU removal and registered callback
which did actual cpu core removali but it was called from
spapr_machine_device_unplug() which is intended for actual object
removal. Commit (cf632463 spapr: Memory hot-unplug support)
sort of fixed it introducing spapr_machine_device_unplug_request()
and calling spapr_core_unplug() but it hasn't renamed callback and
by mistake calls it from spapr_machine_device_unplug().

However spapr_machine_device_unplug() isn't ever called for
cpu core since spapr_core_release() doesn't follow expected
hotunplug call flow which is:
 1: device_del() ->
        hotplug_handler_unplug_request() ->
            set destroy_cb()
 2: destroy_cb() ->
        hotplug_handler_unplug() ->
            object_unparent // actual device removal

Fix it by renaming spapr_core_unplug() to spapr_core_unplug_request()
which is called from spapr_machine_device_unplug_request() and
making spapr_core_release() call hotplug_handler_unplug() which
will call spapr_machine_device_unplug() -> spapr_core_unplug()
to remove cpu core.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reveiwed-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>