rpc: use virNetMessageClearPayload in client

rpc: use virNetMessageClearPayload in client

This removes the opencoded payload freeing in the client, to use
the shared virNetMessageClearPayload call. Two changes:

- ClearPayload sets nfds=0, which fixes a potential crash if
  an error path called virNetMessageFree/Clear on the message
  after fds was free'd
- We drop the inner loop VIR_FORCE_CLOSE... this may mean fds are
  kept open a little bit longer if the call is blocking but in
  practice I don't think it will have any effect