[server management] adding CPU fails when server is shutdown

```
# anvil-shutdown-server --server an-test-deploy1
Are you sure you want to shutdown: [an-test-deploy1]? [y/N]. y
The cluster is up and the node is ready.
The server: [an-test-deploy1] will now be asked to shut down. If the server doesn't stop, please log into it and make sure it reacted to the power button event. Shut it down manually, if needed.
The server: [an-test-deploy1] has shut down.
Done!
```

wait for server to be down.

```
# anvil-manage-server-system --cpu 8 --server an-test-deploy1
Working with the server: [an-test-deploy1], UUID: [427d1401-3302-4679-a998-a05f17c268ef]
The server: [an-test-deploy1] is marked as running, but the host is invalid.
```