[FIX] product: _compute_qty: first round before ceiling, to avoid pathological cases

Fixes problem when we try to sell 12 units of a product and change it to 1 dozen,
the algorithm was then trying to recompute the original amount and was getting
12,0000048 as a result which was then passed to the ceiling method, getting 13.0!

See also previous commit and issue #1125, PR #1126