Device handling API design issues

[marmarta]

Qubes R4.0:

Event 'device-attached' means two essentially different things:

• if a VM is running, it means "a device was successfully attached to a VM and now is in use by that VM"
• if a VM is not running, it means "set the VM to try to use that device whenever it starts".
  Those should be two different events: an actual device-attach and some sort of device-assigned-to-vm, because the current way leads to tears and heartbreak and confusion.

Idea of "persistent" assignment doesn't really fit for some devices. It's okay-ish for PCI, as those devices are not hot-pluggable (mostly), but not so much for USB. The most weirdest is when VM is started with a device attached persistently, but then the device is unplugged. Now device is still "attached persistently", but not really attached (or even present).
There are also other problems like #4780. Also, it's useful to be able to start a VM even if device currently is missing (and attach it when device will be plugged in).

[marmarek]

Those also should be two separate Admin API methods, instead of just persistent option.

[marmarek]

Another issue with the current devices API is how new devices are announced - there is just a device-list-changed:<type> event, but there is no information what change happened. It may be new device, it may be device removal, it maybe some device metadata change etc. Currently it's responsibility of every interested application (devices widget in practice) to refresh the list, compare and act accordingly like show a notification.

[marmarta]

So, I made a proposal for how the new API should look like from the client perspective. Any comments, @marmarek ?