Allow BOM with multiple sizes and virtual stock to count stock by the size grid

Currently virtual stock doesn't count inventory by the size grid, only total stock for the product.

If virtual stock was tracking stock by the size grid it would allow us to accurately track stock for bundles and packs.

For example, if a product has inventory of 9 Small and 11 Large and we are selling a pack of 2 for each size, currently virtual stock would show an inventory of 10 total with no size consideration. Ideally this would show virtual stock of 4 Small and 5 Large for a total of 9.