hbm2hbmxml generates xml for subclasses that doesn't validate under the dtd

When generating xml for subclasses, hbm2hbmxml makes the following errors:

1) A table attribute is always generated, but this is not a valid attribute for the subclass element

2) A discriminator-value attribute is always generated, but this is not a valid attribute for the
union-subclass element.

3) An Id element is generated, but this is not valid for any of the three different kinds of subclass
elemtns.

A test case (test.jar) is attached demonstrating a subclass element thatis generated with a table attribute
and an id element.

A suggested patch is also attached. It entails small changes to persistentclas.hbm.ftl and three
simple helper methods added to Cfg2HbmTool.java to support the persistentclass.hbm.ftl changes.