block/export: only acquire AioContext once for vhost_user_server_stop()

vhost_user_server_stop() uses AIO_WAIT_WHILE(). AIO_WAIT_WHILE()
requires that AioContext is only acquired once.

Since blk_exp_request_shutdown() already acquires the AioContext it
shouldn't be acquired again in vhost_user_server_stop().

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20230323145853.1345527-1-stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

util/vhost-user-server.c

@@ -346,10 +346,9 @@ static void vu_accept(QIONetListener *listener, QIOChannelSocket *sioc,
     aio_context_release(server->ctx);
 }
 
+/* server->ctx acquired by caller */
 void vhost_user_server_stop(VuServer *server)
 {
-    aio_context_acquire(server->ctx);
-
     qemu_bh_delete(server->restart_listener_bh);
     server->restart_listener_bh = NULL;
 
@@ -366,8 +365,6 @@ void vhost_user_server_stop(VuServer *server)
         AIO_WAIT_WHILE(server->ctx, server->co_trip);
     }
 
-    aio_context_release(server->ctx);
-
     if (server->listener) {
         qio_net_listener_disconnect(server->listener);
         object_unref(OBJECT(server->listener));