Merge remote-tracking branch 'remotes/armbru/tags/pull-qapi-2019-09-24' into staging

Merge remote-tracking branch 'remotes/armbru/tags/pull-qapi-2019-09-24' into staging

QAPI patches for 2019-09-24

# gpg: Signature made Tue 24 Sep 2019 13:10:36 BST
# gpg:                using RSA key 354BC8B3D7EB2A6B68674E5F3870B400EB918653
# gpg:                issuer "armbru@redhat.com"
# gpg: Good signature from "Markus Armbruster <armbru@redhat.com>" [full]
# gpg:                 aka "Markus Armbruster <armbru@pond.sub.org>" [full]
# Primary key fingerprint: 354B C8B3 D7EB 2A6B 6867  4E5F 3870 B400 EB91 8653

* remotes/armbru/tags/pull-qapi-2019-09-24: (37 commits)
  qapi: Assert .visit() and .check_clash() run only after .check()
  qapi: Fix excessive QAPISchemaEntity.check() recursion
  qapi: Fix to .check() empty structs just once
  qapi: Delete useless check_exprs() code for simple union kind
  qapi: Clean up around check_known_keys()
  qapi: Simplify check_keys()
  qapi: Normalize 'if' in check_exprs(), like other sugar
  qapi: Fix missing 'if' checks in struct, union, alternate 'data'
  qapi: Reject blank 'if' conditions in addition to empty ones
  qapi: Fix broken discriminator error messages
  qapi: Remove null from schema language
  qapi: Improve reporting of lexical errors
  qapi: Use quotes more consistently in frontend error messages
  tests/qapi-schema: Demonstrate suboptimal lexical errors
  tests/qapi-schema: Demonstrate insufficient 'if' checking
  tests/qapi-schema: Demonstrate broken discriminator errors
  tests/qapi-schema: Demonstrate misleading optional tag error
  tests/qapi-schema: Delete two redundant tests
  tests/qapi-schema: Cover unknown pragma
  qapi: Tweak code to match docs/devel/qapi-code-gen.txt
  ...

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