Split out code for handling incoming method call messages

Split out code for handling incoming method call messages

The remoteDispatchClientRequest() method is currently hardwired to
assume there is only one type of incoming message, a method call.
To allow for alternate types of incoming messags, the code that is
specific to method calls is being split into a separate method
remoteDispatchClientCall

* qemud/dispatch.c: Move method call specific code out into
  remoteDispatchClientCall. Add a helper remoteSerializeError
  for returning error messages to client