CA-396743: forbid setting NBD purpose on unmanaged networks

We wouldn't be able to add the correct firewall rules,
and you're not meant to use the boot from SAN network for NBD.

Signed-off-by: Edwin Török <[email protected]>

ocaml/xapi/xapi_network.ml

@@ -439,6 +439,7 @@ let assert_can_add_purpose ~__context ~network:_ ~current:_ newval =
       assert_no_net_has_bad_porpoise [`nbd]
 
 let add_purpose ~__context ~self ~value =
+  assert_network_is_managed ~__context ~self:value ;
   let current = Db.Network.get_purpose ~__context ~self in
   if not (List.mem value current) then (
     assert_can_add_purpose ~__context ~network:self ~current value ;