Destroy qemu driver under lock (may fix crashes after attach/detach?)

src/client/qemu_driver.c

@@ -568,6 +568,7 @@ static int vitastor_file_open(BlockDriverState *bs, QDict *options, int flags, E
 static void vitastor_close(BlockDriverState *bs)
 {
     VitastorClient *client = bs->opaque;
+    qemu_mutex_lock(&client->mutex);
     vitastor_c_destroy(client->proxy);
     if (client->fds)
     {
@@ -575,7 +576,6 @@ static void vitastor_close(BlockDriverState *bs)
         client->fds = NULL;
         client->fd_alloc = client->fd_count = 0;
     }
-    qemu_mutex_destroy(&client->mutex);
     if (client->config_path)
         g_free(client->config_path);
     if (client->etcd_host)
@@ -586,6 +586,8 @@ static void vitastor_close(BlockDriverState *bs)
         g_free(client->image);
     free(client->last_bitmap);
     client->last_bitmap = NULL;
+    qemu_mutex_unlock(&client->mutex);
+    qemu_mutex_destroy(&client->mutex);
 }
 
 #if QEMU_VERSION_MAJOR >= 3 || QEMU_VERSION_MAJOR == 2 && QEMU_VERSION_MINOR > 2