Do not check host/storage tags on stopped router vm

Error seen:

```
Note: Router r-939-VM has state Stopped
Error: your current cluster has hosts with tags 'systemvm'. Migration will only work when current cluster is in Disabled state and has the host tag 'systemvm' removed.
```

But router is stopped so doesn't make sense. This is due to "false" returned by `checkClusterHostTags`.

Via @fborn 
