function rm_categories_post_count_filter ($cat_post_count) {
    $cat_post_count = str_replace('(', '<span class="post_count pull-right"> (', $cat_post_count);
    $cat_post_count = str_replace(')', ' )</span>', $cat_post_count);
    return $cat_post_count;
}
add_filter('wp_list_categories','rm_categories_post_count_filter');