After doing some RnD, I found out that Windows 8 Server is renamed as "Windows Server 2012" by microsoft, So VirtualMachineGuestOsIdentifier for Windows Server 2012 is "windows8Server64Guest". Please check it on following link,
http://blogs.msdn.com/b/msgulfcommunity/archive/2012/04/19/windows-server-8-is-now-officially-window...