Add new checkbox/pref for disconnecting qube from netvm without setting it to none

[jamke]

The problem you're addressing (if any)

Turning internet connection of qubes on and off is currently complicated and error-prone. E.g. user has qube with sys-vpn netvm, making it offline for a moment requires user to change netvm to none and the information about previous value will be lost.

When user wants to make qube online again, they can easily chose sys-firewall, sys-whonix, sys-othervpn or any other wrong netvm qube.

The solution you'd like

A good approach exists in VirtualBox with Cable connected checkbox.
User can select NAT for interface to provide internet, but manage online/offline status with additional checkbox Cable connected.

In Qubes OS it can be implemented without major difficulties in similar manner.

1. Add a new boolean pref, e.g. connected-to-netvm.
2. Actual connection of netvm should be done only in case new pref is true and netvm is not none.
3. Actual disconnection of netvm should be done in either case netvm is none or new pref is false.

Additional points

• Current default behavior for users will not be affected, because default value will be connected-to-netvm = true.
• Also scripts will not break because netvm contract will be preserved completely.
• qubes-vm-settings should have additional checkbox similar to VirtualBox to make it convenient and transparent for average user.

The value to a user, and who that user might be

1. Convenient switching online/offline for qube, as in VirtualBox.
2. More reliable OFFLINE state of offline qubes like vault, because user can additionally set connected-to-netvm = false and does not worry is at some point netvm will be changed.