Introduces an API for encoding the header field for outgoing messages
allowing some duplicated code to be eliminated

* qemud/dispatch.c, qemud/dispatch.h: add remoteEncodeClientMessageHeader
  for encoding message header. Update remoteDispatchClientRequest to
  use this method.
* qemud/remote.c: Update remoteDispatchDomainEventSend to use the
  generic remoteEncodeClientMessageHeader() for encoding event
  message hedaders. Push some logic from remoteRelayDomainEvent
  down into remoteDispatchDomainEventSend.