Some xml parsing fixes commited

[Štěpán Henek]

Hi,

I committed some fixes for the parsing of the fuzzing xml:
* Fixup and Relation tags are allowed inside the data model
* <!-- --> comments are allowed inside the state and data model

I finished committing for now. So I guess a new installer can be created (After
a proper testing of course ;-))

Cheers,

Stepan