Allow resuming saved machine state when gui is enabled

Attempting to resume a suspended (saved) machine would result in:

```
Call to virDomainUndefine failed: Requested operation is not valid:
Refusing to undefine while domain managed save image exists
(Libvirt::Error)
```

To mitigate this, do not attempt to modify the saved VM when resuming.
Merge pull request #52 from pdf/resume_gui_failure

Allow resuming saved machine state when gui is enabled