Merge remote-tracking branch 'remotes/armbru/tags/pull-monitor-2018-06-18' into staging

Monitor patches for 2018-06-18

# gpg: Signature made Mon 18 Jun 2018 14:50:29 BST
# gpg:                using RSA key 3870B400EB918653
# gpg: Good signature from "Markus Armbruster <armbru@redhat.com>"
# gpg:                 aka "Markus Armbruster <armbru@pond.sub.org>"
# Primary key fingerprint: 354B C8B3 D7EB 2A6B 6867  4E5F 3870 B400 EB91 8653

* remotes/armbru/tags/pull-monitor-2018-06-18:
  monitor: add lock to protect mon_fdsets
  monitor: move init global earlier
  monitor: remove event_clock_type
  monitor: fix comment for monitor_lock
  monitor: more comments on lock-free elements
  monitor: protect mon->fds with mon_lock
  monitor: rename out_lock to mon_lock

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