[FIX] purchase: vendor price not converted to default UoM

-Define "g" as the default UoM for a product.
-Make a purchase order with that product.
-Select the "kg" UoM in the order line and a price of 50.
-Check the Purchase tab of the product.

Before this commit:

the UoM of the price created is the default UoM for the product, "g".
This is because it's a related field.
However, the price wasn't converted to the default UoM, it's still 50.

After this commit:

the price is converted to the default UoM, it's 0.05 per "g".

closes odoo/odoo#38522

Opw: 2076722
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>