The XML generation of service is broken for form types

For a form type, it generates the following snippet:

``` xml
<service class="Incenteev\WebBundle\Form\Type\SuffixedTextType" id="incenteev.form.suffixed_text">
  <tags alias="incenteev_suffixed_text" name="form.type"/>
</service>
```

this is wrong. It should be `<tag>`, not `<tags>`
