Merge remote-tracking branch 'remotes/armbru/tags/pull-qapi-2018-08-28' into staging

QAPI patches for 2018-08-28

# gpg: Signature made Tue 28 Aug 2018 17:23:32 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-qapi-2018-08-28:
  qapi: Add comments to aid debugging generated introspection
  qapi: Minor introspect.py cleanups
  qapi: Update docs for generator changes since commit 9ee86b85267
  qapi: Emit a blank line before dummy declaration
  qapi: Drop qapi_event_send_FOO()'s Error ** argument
  qapi: Fix build_params() for empty parameter list

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