Rework unattended.xml support for Windows 2008 R2

The unattended.xml we use for Windows 2008 needed a few fixups to work
properly when doing image-based installation.  With this set of fixups,
the Windows 2008 install requires zero intervention to get to a login
screen.
Merge pull request #30 from VictorLowther/start-windows-2008-r2-image-install

Start reworking windows 2008 to do image-based installation
Remove hardcoded 2008 R2 Enterprise
Change default to Windows 2008 R2 SERVERSTANDARD.