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 Tue 28 Jun 2016 22:27:20 BST
# gpg:                using RSA key 0x9CA4ABB381AB73C8
# gpg: Good signature from "Stefan Hajnoczi <stefanha@redhat.com>"
# gpg:                 aka "Stefan Hajnoczi <stefanha@gmail.com>"
# Primary key fingerprint: 8695 A8BF D3F9 7CDA AC35  775A 9CA4 ABB3 81AB 73C8

* remotes/stefanha/tags/tracing-pull-request:
  trace: [*-user] Add events to trace guest syscalls in syscall emulation mode
  trace: enable tracing in qemu-img
  qemu-img: move common options parsing before commands processing
  trace: enable tracing in qemu-nbd
  trace: enable tracing in qemu-io
  trace: move qemu_trace_opts to trace/control.c
  doc: move text describing --trace to specific .texi file
  doc: sync help description for --trace with man for qemu.1

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