Merge remote-tracking branch 'remotes/armbru/tags/pull-qapi-2020-07-21' into staging

QAPI patches patches for 2020-07-21

# gpg: Signature made Tue 21 Jul 2020 14:05:05 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-2020-07-21:
  qapi: Fix visit_type_STRUCT() not to fail for null object

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