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
res = super(stock_picking, self).write(cr, uid, ids, vals, context=context)
        after_vals = {}
        if vals.get('location_id'):
            after_vals['location_id'] = vals['location_id']
        if vals.get('location_dest_id'):
            after_vals['location_dest_id'] = vals['location_dest_id']
        
        if after_vals:
            moves = []
            for pick in self.browse(cr, uid, ids, context=context):
                moves += [x.id for x in pick.move_lines if not x.scrapped]
            if moves:
                self.pool['stock.move'].write(cr, uid, moves, after_vals, context=context)
        return res
    def _state_get(self, cr, uid, ids, field_name, arg, context=None):
        '''The state of a picking depends on the state of its related stock.move
            draft: the picking has no line or any one of the lines is draft
            done, draft, cancel: all lines are done / draft / cancel
            confirmed, waiting, assigned, partially_available depends on move_type (all at once or partial)
        '''
        res = {}
        for pick in self.browse(cr, uid, ids, context=context):
            if not pick.move_lines:
                res[pick.id] = pick.launch_pack_operations and 'assigned' or 'draft'
                continue