Impacted versions: 9.0

Steps to reproduce:

* Create an MTO product with 
  * Product Type: Stockable Product
  * Routes: "Buy" and "Make to Order"
* Update Qty on Hand to any quantity (i.e, 2).
* Create a sales order with this product and a number <= to the one in the step before.
* Confirm the sales order.

Current behavior:

The created delivery picking appears in state "Reserved" and the current quantity on stock has been reserved, allowing you to transfer that stock.

Expected behavior:

The created delivery picking should be in "Waiting another operation", or at least in "Waiting availability" until the purchase order is confirmed (if you have defined a supplier for the product and the procurement creates the purchase order), which seems to be the current behavior when there's no stock available (is this also correct, as it allows to manually reserve?).
addons/stock/tests/test_stock_flow.py
self.assertEqual(set(d_done_qty), set([3.0, 5.0]), 'Wrong quantity of moves product D.')
        
        self.assertFalse(self.PickingObj.search([('backorder_id', '=', back_order_in.id)]), "Should not create any back order.")
        
        
        
        
        
        quants = self.StockQuantObj.search([('product_id', '=', self.productA.id), ('location_id', '=', self.stock_location)])
        total_qty = [quant.qty for quant in quants]
        self.assertEqual(sum(total_qty), 12.0, 'Wrong total stock location quantity (%s found instead of 12)' % (sum(total_qty)))
        self.assertEqual(self.productA.qty_available, 12.0, 'Wrong quantity available (%s found instead of 12)' % (self.productA.qty_available))
        
        quants = self.StockQuantObj.search([('product_id', '=', self.productB.id), ('location_id', '=', self.stock_location)])
        self.assertFalse(quants, 'No quant should found as outgoing shipment took everything out of stock')
        self.assertEqual(self.productB.qty_available, 0.0, 'Total quantity in stock should be 0 as the backorder took everything out of stock')
        
        quants = self.StockQuantObj.search([('product_id', '=', self.productC.id), ('location_id', '=', self.stock_location)])
        total_qty = [quant.qty for quant in quants]
        self.assertEqual(sum(total_qty), 8.0, 'Wrong total stock location quantity (%s found instead of 8)' % (sum(total_qty)))
        self.assertEqual(self.productC.qty_available, 8.0, 'Wrong quantity available (%s found instead of 8)' % (self.productC.qty_available))
        
        quants = self.StockQuantObj.search([('product_id', '=', self.productD.id), ('location_id', '=', self.stock_location)])
        total_qty = [quant.qty for quant in quants]
        self.assertEqual(sum(total_qty), 9.0, 'Wrong total stock location quantity (%s found instead of 9)' % (sum(total_qty)))
        self.assertEqual(self.productD.qty_available, 9.0, 'Wrong quantity available (%s found instead of 9)' % (self.productD.qty_available))
        
        
        
        back_order_out.do_prepare_partial()