Wrong filename for 7-zip, wrong path for VBoxGuestAdditions.iso.
Workaround for PROCESSOR_ARCHITECTURE.

Apparently PROCESSOR_ARCHITECTURE is not set while vm-geust-tools.bat is
being executed. Workaround that for now.
Fix for forwarded ports.

Don't set forwarded ports in this base box, because apparently(?) they
can't be overriden anymore in boxes that inherit from this one.