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

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

# gpg: Signature made Wed 03 Feb 2016 15:47:34 GMT using RSA key ID 81AB73C8
# gpg: Good signature from "Stefan Hajnoczi <stefanha@redhat.com>"
# gpg:                 aka "Stefan Hajnoczi <stefanha@gmail.com>"

* remotes/stefanha/tags/tracing-pull-request:
  log: add "-d trace:PATTERN"
  trace: switch default backend to "log"
  trace: convert stderr backend to log
  log: move qemu-log.c into util/ directory
  log: do not unnecessarily include qom/cpu.h
  trace: add "-trace help"
  trace: add "-trace enable=..."
  trace: no need to call trace_backend_init in different branches now
  trace: split trace_init_file out of trace_init_backends
  trace: split trace_init_events out of trace_init_backends
  trace: fix documentation
  trace: track enabled events in a separate array
  trace: count number of enabled events

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