io: check there are no qio_channel_yield() coroutines during ->finalize()

Callers must clean up their coroutines before calling
object_unref(OBJECT(ioc)) to prevent an fd handler leak. Add an
assertion to check this.

This patch is preparation for the fd handler changes that follow.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-ID: <20230830224802.493686-4-stefanha@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>

io/channel.c

@@ -653,6 +653,10 @@ static void qio_channel_finalize(Object *obj)
 {
     QIOChannel *ioc = QIO_CHANNEL(obj);
 
+    /* Must not have coroutines in qio_channel_yield() */
+    assert(!ioc->read_coroutine);
+    assert(!ioc->write_coroutine);
+
     g_free(ioc->name);
 
 #ifdef _WIN32