Restart a running Cube

Cold-restarts the Cube: the hypervisor process is stopped and relaunched, so the Cube boots against the host's current kernel. This is how a Cube picks up a refreshed guest kernel after a platform image update — a reboot issued inside the Cube cannot do it, because the kernel is supplied externally by the host. Disk state is preserved; only the kernel changes. Cube must be running (a stopped Cube already picks up the latest kernel when started). Concurrent restarts of the same Cube are rejected with 409 rather than queued twice.