Improve c1490684b8d32d9c by making the hack more self-contained.

Otherwise, XSD_SomeType (from xsd:SomeType in the schema) would suddenly
become XML_SomeType, due to the hardcoded prefix.
Add server-side test in IHC test (for inheritance support)

Unfinished, but it already caught the XSD_ArrayOfint -> XML_ArrayOfint
regression, so let's have it in.