This fixes several issues:
- Some classes like XMLObject were outside Products.ERP5Type.Document
  and there were not migrated.
- Persistent migration using _delOb/_setOb does not work with mount points.

A new Base.migrateToPortalTypeClass method is also provided to migrate objects
persistently. Note however that migration of HBTrees requires additional work,
using PickleUpdater method.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@44780 20353a03-c40f-0410-a6d1-a30d3c3de9de