Added support for OEM folders on Windows guests

Hi Chris,

As commented previously on aeolu-devel, it could be interesting to have support for $OEM$ folders (http://technet.microsoft.com/en-us/library/dd744265%28v=ws.10%29.aspx) for Windows guests.
The $OEM$ folder is one way of customizing some parameters of Windows installations (drivers, files, etc). This version is based on a new parameter in the TDL: <oemfolder>.

If you need further information, ask me!