Virtual users must be searchable and their rights managed

Virtual users must be searchable. The existing "searchable" property is not used.
Moreover, we must be able to set rights on virtual users.

=> change the usermanager implementation to also look at the virtual users,
=> or implement a virtual directory which will be contributed to the usermanager and work like any other directory.

Also, look at the virtual groups.

Related issues: NXP-2585, NXP-9098