Untangle the XML/Hello message decoders

The tie between these classes was completely unnecessary -- we had this
complicated contract just for the sake of reusing couple of lines of
code. Untangle and optimize.

Change-Id: I6fd29ba0ff5d87a5fb2362e87e4fb1361544429f
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "Untangle the XML/Hello message decoders"