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>