Fix a few errors on preview billing rework

Another WIP commit to sync with laptop, do not forget to rebase!

uncloud/uncloud_vm/views.py

@@ -31,21 +31,15 @@ class VMProductViewSet(ProductViewSet):
         # Create base order.
         order = Order.objects.create(
                 recurring_period=RecurringPeriod.PER_MONTH,
-                recurring_price=0,
-                one_time_price=0,
                 owner=request.user
                 )
+        order.save()
 
         # Create VM.
         serializer = VMProductSerializer(data=request.data, context={'request': request})
         serializer.is_valid(raise_exception=True)
         vm = serializer.save(owner=request.user, order=order)
 
-        # FIXME: commit everything (VM + order) at once.
-        order.recurring_price = vm.recurring_price(order.recurring_period)
-        order.one_time_price = 0
-        order.save()
-
         return Response(serializer.data)