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

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

Pull request

A build fix and a 'simple' trace backend regression fix.

# gpg: Signature made Thu 30 Jul 2020 19:13:10 BST
# gpg:                using RSA key 8695A8BFD3F97CDAAC35775A9CA4ABB381AB73C8
# gpg: Good signature from "Stefan Hajnoczi <stefanha@redhat.com>" [full]
# gpg:                 aka "Stefan Hajnoczi <stefanha@gmail.com>" [full]
# Primary key fingerprint: 8695 A8BF D3F9 7CDA AC35  775A 9CA4 ABB3 81AB 73C8

* remotes/stefanha/tags/tracing-pull-request:
  tracetool: carefully define SDT_USE_VARIADIC
  trace/simple: Allow enabling simple traces from command line

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>