Hello Martin,
Have you tried replacing this:
<cpi:offlineImage cpi:source="wim://win7provm2/users/gowestev/desktop/win7x86.wim#OS" xmlns:cpi="urn:schemas-microsoft-com:cpi" />
with this:
<cpi:offlineImage cpi:source="catalog:y:/sources/install_windows 7 professional.clg" xmlns:cpi="urn:schemas-microsoft-com:cpi" />