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
initial_values = dict((picking.id, {'state': picking.state}) for picking in pickings)
        res = super(stock_move, self).write(cr, uid, ids, vals, context=context)
        if track_pickings:
            picking_obj.message_track(cr, uid, to_track_picking_ids, picking_obj.fields_get(cr, uid, ['state'], context=context), initial_values, context=context)
        return res
    def onchange_quantity(self, cr, uid, ids, product_id, product_qty, product_uom):
        """ On change of product quantity finds UoM
        @param product_id: Product id
        @param product_qty: Changed Quantity of product
        @param product_uom: Unit of measure of product
        @return: Dictionary of values
        """
        warning = {}
        result = {}
        if (not product_id) or (product_qty <= 0.0):
            result['product_qty'] = 0.0
            return {'value': result}
        product_obj = self.pool.get('product.product')
        
        if ids:
            for move in self.read(cr, uid, ids, ['product_qty']):
                if product_qty < move['product_qty']:
                    warning.update({
                        'title': _('Information'),
                        'message': _("By changing this quantity here, you accept the "
                                "new quantity as complete: Odoo will not "
                                "automatically generate a back order.")})
                break
        return {'warning': warning}
    def onchange_product_id(self, cr, uid, ids, prod_id=False, loc_id=False, loc_dest_id=False, partner_id=False):