virtio: make del_vqs idempotent

Our code calls del_vqs multiple times, assuming
it's idempotent.

commit 3ec7a77bb3
    virtio_pci: free up vq->priv
broke this assumption, by adding kfree there,
so multiple calls cause double free.

Fix it up.

Fixes: 3ec7a77bb3
Reported-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_pci_common.c

@@ -282,6 +282,7 @@ void vp_del_vqs(struct virtio_device *vdev)
 
 	vp_free_vectors(vdev);
 	kfree(vp_dev->vqs);
+	vp_dev->vqs = NULL;
 }
 
 static int vp_try_to_find_vqs(struct virtio_device *vdev, unsigned nvqs,