XML: pass XML structure to caller through p_(add|get)_proto_data functions

Since ge3a04bb data parameter is used for the media-type string

Bug: 10729
Change-Id: I3df640079a8bf57f4bd86a1baa08cbf9a3a7e1b3
Reviewed-on: https://code.wireshark.org/review/5511
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
Reviewed-by: Michael Mann <mmann78@netscape.net>