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 location in self.browse(cr, uid, ids, context=context):
            res.append((location.id, self._name_get(cr, uid, location, context=context)))
        return res
    _columns = {
        'name': fields.char('Location Name', required=True, translate=True),
        'active': fields.boolean('Active', help="By unchecking the active field, you may hide a location without deleting it."),
        'usage': fields.selection([
                        ('supplier', 'Vendor Location'),
                        ('view', 'View'),
                        ('internal', 'Internal Location'),
                        ('customer', 'Customer Location'),
                        ('inventory', 'Inventory Loss'),
                        ('procurement', 'Procurement'),
                        ('production', 'Production'),
                        ('transit', 'Transit Location')],
                'Location Type', required=True,
                help="""* Vendor Location: Virtual location representing the source location for products coming from your vendors
                       \n* View: Virtual location used to create a hierarchical structures for your warehouse, aggregating its child locations ; can't directly contain products
                       \n* Internal Location: Physical locations inside your own warehouses,
                       \n* Customer Location: Virtual location representing the destination location for products sent to your customers
                       \n* Inventory Loss: Virtual location serving as counterpart for inventory operations used to correct stock levels (Physical inventories)
                       \n* Procurement: Virtual location serving as temporary counterpart for procurement operations when the source (vendor or production) is not known yet. This location should be empty when the procurement scheduler has finished running.
                       \n* Production: Virtual counterpart location for production operations: this location consumes the raw material and produces finished products
                       \n* Transit Location: Counterpart location that should be used in inter-companies or inter-warehouses operations