tests/qapi-schema: Document events with base don't work

When event FOO's 'data' is a struct with a base, we consider only the
struct's direct members, and ignore its base.  The generated
qapi_event_send_foo() doesn't take arguments for base members.

No such events currently exist in the QMP schema.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>