Ensure empty payload is written upon stream completion

Ensure empty payload is written upon stream completion

On stream completion it is neccessary to send back a
message with an empty payload. The message header was
not being filled out correctly, since we were not writing
any payload. Add a method for encoding an empty payload
which updates the message headers correctly.

* src/rpc/virnetmessage.c, src/rpc/virnetmessage.h: Add
  a virNetMessageEncodePayloadEmpty method
* src/rpc/virnetserverprogram.c: Write empty payload on
  stream completion