vmspawn: by default, let machined register a cgroup for VMs

vmspawn: by default, let machined register a cgroup for VMs

This mimics what we do in nspawn: if registration is enabled we'll let
machined allocate a scope unit for us. When --keep-unit is used we'll
register without creating a new scope.

This brings behaviour more inline with what nspawn does, exposing the
same sets of options.