I have the SAME Exact Problem. Trying to install Astaro Security Gateway in a physically separated Network VM...
The XML gets "appended with a "special" line without any of my action AND the VM won't start.
My cpu supports all the VM shananagans
http://ark.intel.com/products/33910/Intel-Core2-Duo-Processor-E8400-6M-Cache-3_00-GHz-1333-MHz-FSB
Don't know what to do...