GRE isolation + XCP-NG

### problem

The documentation states that GRE is supported for network isolation.

If you specify the tag (as stated in the documentation), you can't select a created Network offering (GRE) when creating a network. If you remove the tag, the network appears, but the virtual router doesn't start.

```
Start Router
(r-51-VM) class com.cloud.agent.api.UnsupportedAnswer cannot be cast to class com.cloud.agent.api.OvsFetchInterfaceAnswer (com.cloud.agent.api.UnsupportedAnswer and com.cloud.agent.api.OvsFetchInterfaceAnswer are in unnamed module of loader 'app')
```

### versions

4.22, XCP-NG 8.3