[FIX] product: default value on required field

The qty fields has become computed in 6.1, and the value set by the user is on
min_qty. Set default value as it is required.

Closes odoo#8561

addons/product/product.py

@@ -901,7 +901,7 @@ def _calc_qty(self, cr, uid, ids, fields, arg, context=None):
         'company_id':fields.many2one('res.company','Company',select=1),
     }
     _defaults = {
-        'qty': lambda *a: 0.0,
+        'min_qty': lambda *a: 0.0,
         'sequence': lambda *a: 1,
         'delay': lambda *a: 1,
         'company_id': lambda self,cr,uid,c: self.pool.get('res.company')._company_default_get(cr, uid, 'product.supplierinfo', context=c),