Not possible in one step unfortunately. Cloning of running machines is only working for repositories that support snapshots in the first place (so no NFS) and you can only live-clone to the same repository. After that you can move the (halted) clone to a different repository. Unfortunately the latter can only be done in the GUI and not the CLI at the moment. If that was possible it would be a great way for an automated backup. I hope this is coming soon though
Additionally, clones are crash consistent on a per-disk basis. If you have a VM set up with three virtual disks and clone it while it's running, the resultant clone will be as though you took a snapshot of one disk, then another disk, then a third disk and created a VM out of them. If you have transactions spanning disks, they will be invalid.