storage: Introduce virStoragePoolObjEndAPI
    
    For now it'll just call the virStoragePoolObjUnlock, but a future
    adjustment will do something different. Since the new API will check
    for a NULL object before the Unlock call, callers no longer need to
    check for NULL before calling.
    
    The virStoragePoolObjUnlock is now private/static to virstorageobj.c
    with a short term forward reference.