What method should I be calling to get the ProductVariantAttribute XML thats in the Nop_ShoppingCartItem table?  I expected to find a method that takes a ProductVariantAttribute as one of the parameters but haven't found one.  

It's easy to see how the structure should be in the database so I can easily do it myself, but there's already code for doing this somewhere.

I see an XmlHelper class and a CheckoutAttributeHelper class, but it's not readily apparent how to get the XML string from product attributes.

Any help is appreciated!