Merge remote-tracking branch 'remotes/armbru/tags/pull-qapi-2021-02-08' into staging

QAPI patches patches for 2021-02-08

# gpg: Signature made Mon 08 Feb 2021 13:54:26 GMT
# 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-2021-02-08:
  qapi: enable strict-optional checks
  qapi: type 'info' as Optional[QAPISourceInfo]
  qapi/gen: Drop support for QAPIGen without a file name
  qapi/commands: Simplify command registry generation
  qapi/gen: Support switching to another module temporarily
  qapi/gen: write _genc/_genh access shims
  qapi: centralize the built-in module name definition
  qapi/gen: Combine ._add_[user|system]_module
  qapi: use './builtin' as the built-in module name
  qapi: use explicitly internal module names
  qapi/gen: Replace ._begin_system_module()
  qapi: centralize is_[user|system|builtin]_module methods
  qapi/gen: inline _wrap_ifcond into end_if()
  qapi/main: handle theoretical None-return from re.match()
  qapi/events: fix visit_event typing
  qapi/commands: assert arg_type is not None

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