Constrain to EBS instances

Actions start/stop only apply to EBS instances, so in order to avoid the logic to deal with resp. exceptions and UI logic, I suggest to restrict the `DescribeInstancesRequest` with a resp. filter:

```cs
var request = new DescribeInstancesRequest().WithFilter(
						new Filter().WithName("root-device-type").WithValue("ebs"));
```

Of course, in the rare case someone is using non EBS instances, them lacking from the UI might be confusing, but I guess it's not worth the hassle to implement conditional UI logic just yet?!