Merge remote-tracking branch 'remotes/armbru/tags/pull-qapi-2017-01-27' into staging

QAPI/QMP patches for 2017-01-27

# gpg: Signature made Fri 27 Jan 2017 07:24:02 GMT
# gpg:                using RSA key 0x3870B400EB918653
# 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-qapi-2017-01-27:
  qmp: Fix argument name in error message of device-list-properties
  qapi: Remove unwanted commas after #optional keyword
  build-sys: Minor qapi doc generation target cleanups

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