This function is not supposed to dispose the element in the case of failure
as the caller is using the elements name in the error message. Also add
some more input parameter checks in the form of g_return_val_if_fail
This uses same approach like in playbin, namely checking for user defined
element, auto{audio,video}{sink,src} and finally DEFAULT_{AUDIO,VIDEO}{SRC,SINK}
defines from config.h.
