pc: Ensure non-zero CPU ref count after attaching to ICC bus

Setting the parent bus of a device increases its ref count, which we
ultimately want to level out. However it is only safe to do so after the
last reference to the device in local code, as qom-set or similar operations
might decrease the ref count.

Therefore move the object_unref() from pc_new_cpu() into its callers.

The APIC operations on the last CPU in pc_cpus_init() are still potentially
insecure, but that is beyond the scope of this code movement.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
