Add support for dependent disks during in-cluster near-live migration - #3794

@presztak I really don't like the added shared/api options. We shouldn't have anything internal be exposed to the public API in that way.

Is there no way for us to detect what we need internally considering all of this is happening with a shared database and with the client user-agent and certificate set to the cluster internal one so we can detect that this isn't a user request but is in fact a cluster-internal operation.

@stgraber Another option I thought, would be to detect that this is a copy request originating from a different node, I mean, an inter-cluster request. However, reusing copy request elsewhere would then cause us to always skip shared volumes.

quick update, on the server side, we're now using migration directly instead of copy. This avoids exposing new external fields related to dependent disk migration, and has reduced the number of commits to 4. I've moved the PR back to draft, since we're still missing support for near-live migration when the remote is on shared storage but dependent disks are local.

@presztak what's with that "test" commit? :)

@stgraber it's just for testing purpose for now as I've hit issues on linstor driver.

@presztak the Info flag seems wrong to me. It's meant to indicate whether an efficient transfer of incremental state is available or not (the requirement for near live migration).

Only ZFS and btrfs offer that. The rest all rely on rsync/dd which aren't suitable for this use case.

Remote pools within a cluster also don't need the flag set to true anyway since the data isn't going to be moving at all. But if you were to go cross-cluster between remote storage drivers on both ends, none of our drivers have an efficient way to transfer those snapshots so they shouldn't be treated as handling near live migration.

@stgraber, ok, I did it that way because the initial condition was:

if poolInfo.Name == "dir" || poolInfo.Name == "lvm" {
    return fmt.Errorf("Near-live migration isn't supported for dependent disk %q on %q storage pools", dev.Name, poolInfo.Name)
}

So NearLiveMigration was true for everything except dir and lvm. But I see your point now, and I've made adjustments based on your feedback.