Perform a marshalling from the invocator provided (method_call) into the appropiate XML-RPC representation.
This function allows to convert the given method call into the XML-RPC representation. The function return the xml stream representing the invocator in a dynamically allocated value. It should be deallocated once no longer needed.
Additionally, the function allows to set a pointer to a integer (size parameter) so the function could return the xml stream size generated. This method is preferred over getting the xml stream and then using something similar to strlen.