Add quantity to add_to_cart shortcode.

quantity.php

<?php

add_action( 'wpsc_add_to_cart_button_form_end', function( $product_id ) {
?>
<fieldset><legend><?php _e('Quantity', 'wpsc'); ?></legend>
		<div class="wpsc_quantity_update">
		<input type="text" id="wpsc_quantity_update_<?php echo $product_id; ?>" name="wpsc_quantity_update" size="2" value="1" />
								<input type="hidden" name="key" value="<?php echo wpsc_the_cart_item_key(); ?>"/>
								<input type="hidden" name="wpsc_update_quantity" value="true" />
								<input type='hidden' name='wpsc_ajax_action' value='wpsc_update_quantity' />
   </div><!--close wpsc_quantity_update-->
</fieldset>

<?php
} );