[webui] improve live_build_log's stability

There was one long standing issue with the ruby 1.9 port that
chunk.length became longer through the escaping, which lead to the
problem that _log?start=$NEWOFFSET returned "log file not that long",
which then halted the whole update process

Now activexml stores the raw length of the http response, so I can
continue from there. This is a rather non-OOM, but I couldn't think of
a cleaner way