XML export driver: fix the options export

Currently `Doctrine\ORM\Tools\Export\Driver\XmlExporter` produce next result for the field options (example for `default => 1`):
```xml
<options default="1" />
```

but `Doctrine\ORM\Mapping\Driver\XmlDriver` expect:
```xml
<options>
  <option name="default">1</option>
</options>
```

This pull request should fix it :wink: