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

# gpg: Signature made Fri 09 Oct 2015 10:15:13 BST using RSA key ID 81AB73C8
# gpg: Good signature from "Stefan Hajnoczi <[email protected]>"
# gpg:                 aka "Stefan Hajnoczi <[email protected]>"

* remotes/stefanha/tags/tracing-pull-request:
  trace: remove malloc tracing
  docs: update the usage example of "dtrace" backend in tracing.txt

Signed-off-by: Peter Maydell <[email protected]>