feat(live): Improve printing the Agama URL in the console

[lslezak]

Improvements

• Hide Avahi/mDNS URL if network is not configured (displaying https://agama.local URL is confusing if the network is not available, in that case the URL simply cannot work)
• Display only configured interfaces (original code printed empty https:// URL for not configured devices, like not configured/not connected Wifi adapters)
• Refresh the URLs when a device is changed (it also detects cable connecting/disconnecting)
• Display a warning if the network is not available. Originally I wanted to display some hint like "Press Alt+F7 to switch to the local installer" but that does not work on some architectures (S390) or in some situations (serial console). So to avoid confusion I have dropped that feature.

Notes

• Use the NetworkManager backend instead of udev. Udev works on the hardware level and does not detect network configuration changes (configuring Wifi) or changing the connection (unplugging/plugging in the network cable). The NetworkManager provides more details.

Testing

• Tested manually

Screenshots

• Video recording of a session where the virtual network cable in a VirtualBox VM was disconnected and then reconnected again

Agama-screen0.webm

[dgdavid]

LGTM, 👌