Enable sysprep for Windows 2016 LibVirt build.

Packer/windows_2016.json

@@ -93,7 +93,8 @@
       "winrm_username": "vagrant",
       "winrm_password": "vagrant",
       "winrm_timeout": "4h",
-      "shutdown_command": "shutdown /s /t 10 /f /d p:4:1 /c \"Packer Shutdown\"",
+      "shutdown_command": "a:/sysprep.bat",
+      "shutdown_timeout": "2h",
       "accelerator": "kvm",
       "disk_size": "{{user `disk_size`}}",
       "output_directory": "{{ user `packer_build_dir`}}",
@@ -177,4 +178,4 @@
     "autounattend_virtio": "./answer_files/2016_virtio/Autounattend.xml",
     "packer_build_dir": "./win2016"
   }
-}
+}