vdpa_sim: use max_iotlb_entries as a limit in vhost_iotlb_init

Commit bda324fd ("vdpasim: control virtqueue support") changed
the allocation of iotlbs calling vhost_iotlb_init() for each address
space, instead of vhost_iotlb_alloc().

With this change we forgot to use the limit we had introduced with
the `max_iotlb_entries` module parameter.

Fixes: bda324fd ("vdpasim: control virtqueue support")
Cc: gautam.dawar@xilinx.com
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20220621151208.189959-1-sgarzare@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Eugenio Pérez <eperezma@redhat.com>