virtio: add unlikely() to WARN_ON_ONCE()

The condition to test is unlikely() to be true. Add the hint.

Signed-off-by: Igor Stoppa <igor.stoppa@huawei.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Virtualization@lists.linux-foundation.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>