vdpa_sim: not reset state in vdpasim_queue_ready

vdpasim_queue_ready calls vringh_init_iotlb, which resets split indexes.
But it can be called after setting a ring base with
vdpasim_set_vq_state.

Fix it by stashing them. They're still resetted in vdpasim_vq_reset.

This was discovered and tested live migrating the vdpa_sim_net device.

Fixes: 2c53d0f6 ("vdpasim: vDPA device simulator")
Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Message-Id: <20230118164359.1523760-2-eperezma@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Tested-by: Lei Yang <leiyang@redhat.com>

drivers/vdpa/vdpa_sim/vdpa_sim.c

@@ -66,6 +66,7 @@ static void vdpasim_vq_notify(struct vringh *vring)
 static void vdpasim_queue_ready(struct vdpasim *vdpasim, unsigned int idx)
 {
 	struct vdpasim_virtqueue *vq = &vdpasim->vqs[idx];
+	uint16_t last_avail_idx = vq->vring.last_avail_idx;
 
 	vringh_init_iotlb(&vq->vring, vdpasim->features, vq->num, false,
 			  (struct vring_desc *)(uintptr_t)vq->desc_addr,
@@ -74,6 +75,7 @@ static void vdpasim_queue_ready(struct vdpasim *vdpasim, unsigned int idx)
 			  (struct vring_used *)
 			  (uintptr_t)vq->device_addr);
 
+	vq->vring.last_avail_idx = last_avail_idx;
 	vq->vring.notify = vdpasim_vq_notify;
 }