Detect a running VM before creating a backup in the GUI

[Nebucatnetzer]

When a VM is running during backup it can get into a broken state. Therefore a VM should be shutdown or suspended before taking a backup.
It would be good if Borg-Qt informs the user about running VMs so that he can put it into a save state if he decides to do so.
It would probably be a good idea to make this an optional feature. Meaning that the user can deactivate it in the settings.