Merge remote-tracking branch 'remotes/armbru/tags/pull-qapi-2020-09-03' into staging

QAPI patches patches for 2020-09-03

# gpg: Signature made Thu 03 Sep 2020 09:00:37 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-2020-09-03:
  docs/qdev-device-use: Don't suggest -drive and -net can do USB
  qapi: Document event VSERPORT_CHANGE is rate-limited
  docs/interop/qmp-spec: Point to the QEMU QMP reference manual
  scripts/qmp/qom-fuse: Fix getattr(), read() for files in /
  scripts/qmp/qom-fuse: Port to current Python module fuse
  scripts/qmp/qom-fuse: Unbreak import of QEMUMonitorProtocol
  qapi/block-core.json: Remove stale description of 'blockdev-add'
  qapi: enable use of g_autoptr with QAPI types

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