Document potential issue with PowerShell progress bar
Document potential issue with PowerShell progress bar

Since PowerShell subprocesses don't inherit the setting for
`$ProgressPreference`, it's probably a good idea to set this variable to
`SilentlyContinue` globally when using the WinSSH communicator.

This commit also fixes a formatting issues with the list of supported
Windows guests.

sni/vagrant,mitchellh/vagrant,sni/vagrant,sni/vagrant,marxarelli/vagrant,chrisroberts/vagrant,chrisroberts/vagrant,marxarelli/vagrant,sni/vagrant,mitchellh/vagrant,marxarelli/vagrant,mitchellh/vagrant,mitchellh/vagrant,marxarelli/vagrant,chrisroberts/vagrant,chrisroberts/vagrant