spapr: cpu core: separate child threads destruction from machine state operations

spapr: cpu core: separate child threads destruction from machine state operations

Split off destroying VCPU threads from drc callback
spapr_core_release() into new spapr_cpu_core_unrealizefn()
which takes care of internal cpu core state cleanup (i.e.
VCPU threads) and is called when object_unparent(core)
is called.

That leaves spapr_core_release() only with board mgmt
code, which will be moved to board related file in
follow up patch along with the rest on hotplug callbacks.

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