spapr: prevent QEMU crash when CPU realization fails

ICPState objects were being allocated before CPU thread realization.
However commit 9ed656631d73 (xics: setup cpu at realize time) reversed it
by allocating ICPState objects after CPU thread is realized. But it
didn't take care to fix the error path because of which we observe
a SIGSEGV when CPU thread realization fails during cold/hotplug.

Fix this by ensuring that we do object_unparent() of ICPState object
only in case when is was created earlier.

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