Added Windows 8.1 to known OS types (as Windows81, Windows81_64)
Fix transferring files to Windows guests (wincp)

The built-in "wincp" method to transfer a file to a Windows guest
did not work for me. The problem was twofold: First, the URLs were
constructed as http://10.0.2.2:7700c:/Somewhere/Something, note
missing slash after the port number, on which vscript choked.

And second, the built-in HTTP web server was not able to serve the
file at all; it tried to prepend a slash even though the path was
absolute in the Windows sense, i.e. with a drive letter.