Merge remote-tracking branch 'remotes/armbru/tags/pull-qapi-2021-09-25-v2' into staging

Merge remote-tracking branch 'remotes/armbru/tags/pull-qapi-2021-09-25-v2' into staging

QAPI patches patches for 2021-09-25

# gpg: Signature made Mon 27 Sep 2021 13:44:23 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-2021-09-25-v2: (25 commits)
  tests/qapi-schema: Make test-qapi.py -u work when files are absent
  tests/qapi-schema: Use Python OSError instead of outmoded IOError
  test-clone-visitor: Correct an accidental rename
  tests/qapi-schema: Rename flat-union-* test cases to union-*
  qapi: Drop simple unions
  tests/qapi-schema: Purge simple unions from tests
  tests/qapi-schema: Drop simple union __org.qemu_x-Union1
  test-clone-visitor: Wean off __org.qemu_x-Union1
  tests/qapi-schema: Rewrite simple union TestIfUnion to be flat
  tests/qapi-schema: Simple union UserDefListUnion is now unused, drop
  tests/qapi-schema: Wean off UserDefListUnion
  test-clone-visitor: Wean off UserDefListUnion
  test-qobject-output-visitor: Wean off UserDefListUnion
  test-qobject-input-visitor: Wean off UserDefListUnion
  tests/qapi-schema: Prepare for simple union UserDefListUnion removal
  qapi: Convert simple union TransactionAction to flat one
  qapi: Convert simple union ImageInfoSpecific to flat one
  qapi: Convert simple union SocketAddressLegacy to flat one
  qapi: Convert simple union ChardevBackend to flat one
  qapi: Convert simple union MemoryDeviceInfo to flat one
  ...

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