Allow backing up running VMs by pausing or snapshotting to enable automatic nightly backups

[qubesuser]

Currently the Qubes backup system cannot back up running VMs since copying a mounted block device would result in arbitrary corruption regardless of journaling.

This is a big shortcoming because it means that it's impossible to setup automatic nightly backup in a way that doesn't negatively affect the user by shutting down all VMs.

However, if the VM is paused, then the block device can be copied in a consistent way, and the state after restoring will be as if the VM had crashed at that moment, which assuming a journaling filesystem is usually an acceptable result.

Pausing is less disruptive than shutting down, but still problematic (kills network connections, pauses computation), so an usually better solution is to take a snapshot of the device and back up the snapshot, although this requires changes in the storage system to allow taking DM snapshots of anything (and potentially requires more storage than pausing).

[marmarek]

I think this somehow overlaps with #858. Especially when considering creating a backup from inside of VM.

[andrewdavidwong]

Discussion: https://groups.google.com/d/msgid/qubes-devel/20170221212458.GM1146%40mail-itl

[iamahuman]

For running VMs, we can just snapshot the entire domain state (especially if it has no devices attached). However freeze-to-disk feature appears to be "won't do"—see #832 (comment). Also compatibility issues.