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?).
res = []
        for route in self.browse(cr, uid, ids, context=context):
            res += [x.id for x in route.push_ids]
        return res
    _columns = {
        'name': fields.char('Operation Name', required=True),
        'company_id': fields.many2one('res.company', 'Company'),
        'route_id': fields.many2one('stock.location.route', 'Route'),
        'location_from_id': fields.many2one('stock.location', 'Source Location', ondelete='cascade', select=1, required=True,
                                            help="This rule can be applied when a move is confirmed that has this location as destination location"),
        'location_dest_id': fields.many2one('stock.location', 'Destination Location', ondelete='cascade', select=1, required=True,
                                            help="The new location where the goods need to go"),
        'delay': fields.integer('Delay (days)', help="Number of days needed to transfer the goods"),
        'picking_type_id': fields.many2one('stock.picking.type', 'Picking Type', required=True,
                                           help="This is the picking type that will be put on the stock moves"),
        'auto': fields.selection(
            [('auto','Automatic Move'), ('manual','Manual Operation'),('transparent','Automatic No Step Added')],
            'Automatic Move',
            required=True, select=1,
            help="The 'Automatic Move' / 'Manual Operation' value will create a stock move after the current one.  " \
                 "With 'Automatic No Step Added', the location is replaced in the original move."
            ),
        'propagate': fields.boolean('Propagate cancel and split', help='If checked, when the previous move is cancelled or split, the move generated by this move will too'),
        'active': fields.boolean('Active'),
        'warehouse_id': fields.many2one('stock.warehouse', 'Warehouse'),
        'route_sequence': fields.related('route_id', 'sequence', string='Route Sequence',