[3.1.1 beta] Don't auto-delete VM shortcuts on removable drives

[ogerardin]

The current behaviour in 3.1.1 is to automatically and silently delete VM shortcuts at startup when the target is not available.
This is a problem if the target is on a removable drive that is not connected when UTM is started, as you need to manually reimport all the VMs (which can be quite tedious if you have many, see #3685)

Suggestion:

• do not auto-delete shortcuts when the target is not available
• instead add a status "unavailable" with a visual indicator (such as a red cross)
• recheck the shortcuts either periodically, or when an external drive is mounted, or when the user tries to start the VM

Note: this is how VMware for example behaves with unavailable VMs