[Product] Fixed traceback on calculation of cost price.

Hello,

I have fixed the traceback : Create new product and its bom(assign variant in bom) now, go to variant and try to compute cost price it gives traceback.

More details:

- Task : https://www.odoo.com/web#id=8652&view_type=form&model=project.task&menu_id=3942&action=327
- Pad: https://pad.odoo.com/p/r.4Rw1BsnEJjDbTb8i

Thanks,
cod-odoo
It seems that v8 brings a new distinction: boms for end products and bom lines for composants, so you should search here for bom lines