Merge remote-tracking branch 'stefanha/tracing' into staging

# By Markus Armbruster
# Via Stefan Hajnoczi
* stefanha/tracing:
  trace: Fix location of simpletrace.py in docs
  trace: Clean up the "try to update atomic until it worked" loops
  trace: Direct access of atomics is verboten, use the API
  trace: Fix simple trace dropped event record for big endian