spapr: Simplify unplug path

spapr: Simplify unplug path

spapr_lmb_release() and spapr_core_release() call hotplug_handler_unplug()
which after a bunch of indirection calls spapr_memory_unplug() or
spapr_core_unplug().  But we already know which is the appropriate thing
to call here, so we can just fold it directly into the release function.

Once that's done, there's no need for an hc->unplug method in the spapr
machine at all: since we also have an hc->unplug_request method, the
hotplug core will never use ->unplug.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Greg Kurz <groug@kaod.org>
Tested-by: Daniel Barboza <danielhb@linux.vnet.ibm.com>