Fixed issue 3816

Elevated command line is now rendered to a script which is uploaded to the guest and executed. This allows the command line itself to be less than 100 chars to start the script and any user commands are puts into the script which has unlimited* length.
Merge pull request #3847 from mitchellh/command-line-too-long-fix-for-winrm

guests/windows: fix #3816 long arg list