Shorter Windows and VirtualBox version detection

Ordinarily I'd hide these by moving them into a separate method, but
the way Vagrantfiles are executed makes that hard. I'd rather the reader's
attention flow past these helper methods to the configuration options, so I
used dirty Ruby tricks to make them more concise.

Change-Id: I4af087bb022aae84df3c687aba0f65de6f1c4bfb