make sure that external callers won't try to modify
possible_cpus and owner of possible_cpus can access
it directly when it modifies it.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <1484759609-264075-5-git-send-email-imammedo@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>