Replace SimpleRPM with RPMLibraryAdapter

By using the real RPM library, we'll get macro expansion. Also replace
the '@VERSION@' tags with 'UNRELEASED'

related to xenserver/planex#53
planex-clone looks into .spec files as well

An additional workaround was needed, because BuildRequires(pre) resulted
an error message, rpm library did not understand that.

related to xenserver/planex#53