qapi/gen: Support switching to another module temporarily

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20210201193747.2169670-13-jsnow@redhat.com>
[Commit message tweaked]

scripts/qapi/gen.py

@@ -290,6 +290,13 @@ def _add_module(self, name: str, blurb: str) -> None:
         self._module[name] = (genc, genh)
         self._current_module = name
 
+    @contextmanager
+    def _temp_module(self, name: str) -> Iterator[None]:
+        old_module = self._current_module
+        self._current_module = name
+        yield
+        self._current_module = old_module
+
     def write(self, output_dir: str, opt_builtins: bool = False) -> None:
         for name in self._module:
             if QAPISchemaModule.is_builtin_module(name) and not opt_builtins: