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

Pull request

# gpg: Signature made Thu 30 Jan 2020 21:38:06 GMT
# 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:
  qemu_set_log_filename: filename argument may be NULL
  hw/display/qxl.c: Use trace_event_get_state_backends()
  memory.c: Use trace_event_get_state_backends()
  docs/devel/tracing.txt: Recommend only trace_event_get_state_backends()
  Makefile: Keep trace-events-subdirs ordered

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