storage: Introduce virStoragePoolObjEndAPI

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.