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/stock.py
""" Move all non-done lines into a new backorder picking. If the key 'do_only_split' is given in the context, then move all lines not in context.get('split', []) instead of all non-done lines.
        """
        if not backorder_moves:
            backorder_moves = picking.move_lines
        backorder_move_ids = [x.id for x in backorder_moves if x.state not in ('done', 'cancel')]
        if 'do_only_split' in context and context['do_only_split']:
            backorder_move_ids = [x.id for x in backorder_moves if x.id not in context.get('split', [])]
        if backorder_move_ids:
            backorder_id = self.copy(cr, uid, picking.id, {
                'name': '/',
                'move_lines': [],
                'pack_operation_ids': [],
                'backorder_id': picking.id,
            })
            backorder = self.browse(cr, uid, backorder_id, context=context)
            self.message_post(cr, uid, picking.id, body=_("Back order <em>%s</em> <b>created</b>.") % (backorder.name), context=context)
            move_obj = self.pool.get("stock.move")
            move_obj.write(cr, uid, backorder_move_ids, {'picking_id': backorder_id}, context=context)
            if not picking.date_done:
                self.write(cr, uid, [picking.id], {'date_done': time.strftime(DEFAULT_SERVER_DATETIME_FORMAT)}, context=context)
            self.action_confirm(cr, uid, [backorder_id], context=context)
            self.action_assign(cr, uid, [backorder_id], context=context)
            return backorder_id
        return False
    @api.cr_uid_ids_context
    def recheck_availability(self, cr, uid, picking_ids, context=None):