spapr: drop reference on child object during core realization

When a core is being realized, we create a child object for each thread
of the core.

The child is first initialized with object_initialize() which sets its ref
count to 1, and then added to the core with object_property_add_child()
which bumps the ref count to 2.

When the core gets released, object_unparent() decreases the ref count to 1,
and we g_free() the object: we hence loose the reference on an unfinalized
object. This is likely to cause random crashes.

Let's drop the extra reference as soon as we don't need it, after the
thread is added to the core.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
