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

Pull request

# gpg: Signature made Tue 27 Aug 2019 21:33:15 BST
# gpg:                using RSA key 8695A8BFD3F97CDAAC35775A9CA4ABB381AB73C8
# gpg: Good signature from "Stefan Hajnoczi <[email protected]>" [full]
# gpg:                 aka "Stefan Hajnoczi <[email protected]>" [full]
# Primary key fingerprint: 8695 A8BF D3F9 7CDA AC35  775A 9CA4 ABB3 81AB 73C8

* remotes/stefanha/tags/tracing-pull-request:
  trace: Clarify DTrace/SystemTap help message

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