[FIX] stock: incorrect ordered quantity

- Create a stockable product, get 61 units in stock
- Create a SO of 75 units, validate
- In the picking, transfer only 50 units, validate and create a
  backorder
- Print the Delivery Slip of the picking: it shows 61 units ordered
- Print the Delivery Slip of the backorder: it shows 61 units ordered
- Receive the 14 missing units, recheck availability on the backorder:
  it shows 25 units ordered

The quantity ordered should be recomputed from the stock moves when a
partial transfer is done.

opw-747983