Updated guest OS type for Virtualbox / Windows 8.1 joefitzgerald/packer-windows#81

windows_81.json

@@ -41,7 +41,7 @@
       "ssh_password": "vagrant",
       "ssh_wait_timeout": "2h",
       "shutdown_command": "shutdown /s /t 10 /f /d p:4:1 /c \"Packer Shutdown\"",
-      "guest_os_type": "Windows2012_64",
+      "guest_os_type": "Windows81_64",
       "disk_size": 61440,
       "floppy_files": [
         "./answer_files/81/Autounattend.xml",