vhost/test: fix build for vhost test

Since below commit, callers need to specify the iov_limit in
vhost_dev_init() explicitly.

Fixes: b46a0bf7 ("vhost: fix OOB in get_rx_bufs()")
Cc: stable@vger.kernel.org
Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>