Merge remote-tracking branch 'public/tags/tracing-pull-request' into staging

# gpg: Signature made Tue 08 Nov 2016 06:17:13 PM GMT
# gpg:                using RSA key 0x9CA4ABB381AB73C8
# gpg: Good signature from "Stefan Hajnoczi <[email protected]>"
# gpg:                 aka "Stefan Hajnoczi <[email protected]>"
# Primary key fingerprint: 8695 A8BF D3F9 7CDA AC35  775A 9CA4 ABB3 81AB 73C8

* public/tags/tracing-pull-request:
  docs/tracing.txt: Update documentation of default backend

Message-id: 1478629053[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>