After the final reboot and just before the graceful shutdown for the
snapshot, sample the guest's idle CPU and used-RAM percentages (CIM
Win32_PerfFormattedData — locale-independent — averaged over ~5s after a
settle delay). If either exceeds its threshold, abort prepare without
taking the snapshot, so a busy/leaky template isn't frozen into every clone.
New options: --max-idle-cpu (10%), --max-idle-ram (50%), --idle-settle
(30s), --skip-idle-check.