[FIX] stock_account: Return product with revaluation layer

How to reproduce:
1- Create Product Category "PC" AVCO|FIFO Automated.
2- Create Storable Product "P" with product category PC and unit cost to 1.
3- Create SO with 10 units of P.
4- Create PO with 10 units of P and unit cost of 2.
    Delivery of the SO should now have 2 IV layers, with one being "Revaluation of XXX (negative inventory)"
    Global IV of P should be 0 qty, 0 value.
5- Return the delivery order of SO.
6- Open valuation of the return

Error: The value is 0, should be 20
In `_get_price_unit`, `price_unit` is taken on the last svl of the origin_returned_move_id, in our case it's the revaluation layer.
`_get_price_unit` now take all svls of origin_returned_move_id into account.

OPW-2784033

closes odoo/odoo#88110

X-original-commit: f8e2f2a4
Signed-off-by: Arnold Moyaux <arm@odoo.com>