vdpasim: allow to enable a vq repeatedly

Code must be resilient to enable a queue many times.

At the moment the queue is resetting so it's definitely not the expected
behavior.

v2: set vq->ready = 0 at disable.

Fixes: 2c53d0f6 ("vdpasim: vDPA device simulator")
Cc: stable@vger.kernel.org
Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Message-Id: <20220519145919.772896-1-eperezma@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>

drivers/vdpa/vdpa_sim/vdpa_sim.c

@@ -364,11 +364,14 @@ static void vdpasim_set_vq_ready(struct vdpa_device *vdpa, u16 idx, bool ready)
 {
 	struct vdpasim *vdpasim = vdpa_to_sim(vdpa);
 	struct vdpasim_virtqueue *vq = &vdpasim->vqs[idx];
+	bool old_ready;
 
 	spin_lock(&vdpasim->lock);
+	old_ready = vq->ready;
 	vq->ready = ready;
-	if (vq->ready)
+	if (vq->ready && !old_ready) {
 		vdpasim_queue_ready(vdpasim, idx);
+	}
 	spin_unlock(&vdpasim->lock);
 }