Currently not able to receive NFS callbacks

[Peter-JanGootzen]

Because of limitations in libnfs and the absence of a need for NFS callbacks support, virtionfs currently doesn't support anything to do with NFS callbacks. NFS callbacks allow the server to notify or ask the client (of) something, mainly used for cache invalidation and delegations (when a file resides on a client). See the NFS 4, 4.1 and 4.2 RFCs for all the callbacks.