Sudo Slider

function cor_etalage_add_shortcode() {
?>
<div id="etalage"><ul>
<?php
    $the_query = new WP_Query(array(
        'post_type' => array(
            'portfolio'
        )
    ));
    while ($the_query->have_posts()) : $the_query->the_post();
?>
<li><?php the_post_thumbnail('etalage'); ?></li>
<?php endwhile; ?>
<?php wp_reset_postdata(); ?>
</ul></div>
<?php
}
add_shortcode('etalage', 'cor_etalage_add_shortcode');