vhost-user: Fix out of order vring host notification handling

vhost backend sends host notification for every VQ. If backend creates
VQs in parallel, the VHOST_USER_SLAVE_VRING_HOST_NOTIFIER_MSG may
arrive to QEMU in different order than incremental queue index order.

For example VQ 1's message arrive earlier than VQ 0's:
After alloc VhostUserHostNotifier for VQ 1. GPtrArray becomes

    [ nil, VQ1 pointer ]

After alloc VhostUserHostNotifier for VQ 0. GPtrArray becomes

    [ VQ0 pointer, nil, VQ1 pointer ]

This is wrong. fetch_notifier will return NULL for VQ 1 in
vhost_user_get_vring_base, causes host notifier miss removal(leak).

The fix is to remove current element from GPtrArray, make the right
position for element to insert.

Fixes: 503e355 ("virtio/vhost-user: dynamically assign VhostUserHostNotifiers")
Signed-off-by: Yajun Wu <yajunw@nvidia.com>
Acked-by: Parav Pandit <parav@nvidia.com>

Message-Id: <20221018023651.1359420-1-yajunw@nvidia.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/virtio/vhost-user.c

@@ -1593,6 +1593,11 @@ static VhostUserHostNotifier *fetch_or_create_notifier(VhostUserState *u,
 
     n = g_ptr_array_index(u->notifiers, idx);
     if (!n) {
+        /*
+         * In case notification arrive out-of-order,
+         * make room for current index.
+         */
+        g_ptr_array_remove_index(u->notifiers, idx);
         n = g_new0(VhostUserHostNotifier, 1);
         n->idx = idx;
         g_ptr_array_insert(u->notifiers, idx, n);