Merge remote-tracking branch 'remotes/armbru/tags/pull-qapi-2020-04-30' into staging

QAPI patches for 2020-04-30

# gpg: Signature made Thu 30 Apr 2020 06:29:18 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-2020-04-30:
  qapi: Generate simpler marshalling code when no arguments
  qapi: Disallow qmp_marshal_FOO(NULL, ...)
  qom: Simplify object_property_get_enum()
  qapi: Only input visitors can actually fail
  qapi: Assert non-input visitors see only valid alternate tags
  qapi: Clean up visitor's recovery from input with invalid type
  qapi: Assert non-input visitors see only valid narrow integers
  qapi: Assert output visitors see only valid enum values
  qapi: Fix Visitor contract for start_alternate()
  qapi: Assert incomplete object occurs only in dealloc visitor
  qapi: Polish prose in visitor.h
  qapi: Document @errp usage more thoroughly in visitor.h
  qapi: Fix typo in visit_start_list()'s contract
  qapi: Fix the virtual walk example in visitor.h's big comment
  qapi: Belatedly update visitor.h's big comment for QAPI modules
  qemu-option: Clean up after the previous commit
  qobject: Eliminate qdict_iter(), use qdict_first(), qdict_next()
  qobject: Eliminate qlist_iter(), use QLIST_FOREACH_ENTRY() instead
  qobject: Factor out helper json_pretty_newline()
  qobject: Clean up QLIST_FOREACH_ENTRY()

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