- make rollback command print verbose messages

OK, in general this duplication is often solved by special stream that do nothing or that do something depending on passed variable like add http://stackoverflow.com/questions/11826554/standard-no-op-output-stream
and then use it like
```
NullStream null;
stream verbose = quiet ? null : cout;
```
and then use just `verbose << "_("Creating read-only snapshot of default subvolume.") << flush;`