Automatically detect new devices

[joseivanlopez]

Similarly to YaST, Agama uses a "probing" approach to detect the system devices (storage, network, etc). That implies the changes (e.g., plugged/unplugged devices) are not automatically detected. To "see" the new state, Agama has to reprobe again.

Changes in the devices could ruin the installation. For example, when the selected disk disappears. To avoid that, Agama should listen to changes in the system and notifiy about it.