vdpa/mlx5: preserve CVQ vringh index

mlx5_vdpa does not preserve userland's view of vring base for the control
queue in the following sequence:

ioctl VHOST_SET_VRING_BASE
ioctl VHOST_VDPA_SET_STATUS VIRTIO_CONFIG_S_DRIVER_OK
  mlx5_vdpa_set_status()
    setup_cvq_vring()
      vringh_init_iotlb()
        vringh_init_kern()
          vrh->last_avail_idx = 0;
ioctl VHOST_GET_VRING_BASE

To fix, restore the value of cvq->vring.last_avail_idx after calling
vringh_init_iotlb.

Fixes: 5262912e ("vdpa/mlx5: Add support for control VQ and MAC setting")

Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Acked-by: Eugenio Pérez <eperezma@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <1699014387-194368-1-git-send-email-steven.sistare@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>