virNetworkObjListPtr: Make APIs self-locking
    
    Every API that touches internal structure of the object must lock
    the object first. Not every API that has the object as an
    argument needs to do that though. Some APIs just pass the object
    to lower layers which, however, must lock the object then. Look
    at the code, you'll get my meaning soon.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>