dmi.product.uuid should return the system UUID on Windows

On Linux the dmi.product.uuid fact returns a UUID of the system. This value is not provided by Facter for Windows systems, but it is available and should be returned as it is often an important unique identifier for the system.

It is available as the 'UUID' property on the WMI ComputerSystemProduct class (which is already accessed for retrieving the dmi.product.name fact).