There is a very low probability that hitting physical NVMe disk
hardware critical warning case, it's hard to write & test a monitor
agent service.

For debugging purposes, add a new 'smart_critical_warning' property
to emulate this situation.

The orignal version of this change is implemented by adding a fixed
property which could be initialized by QEMU command line. Suggested
by Philippe & Klaus, rework like current version.

Test with this patch:
1, change smart_critical_warning property for a running VM:
 #virsh qemu-monitor-command nvme-upstream '{ "execute": "qom-set",
  "arguments": { "path": "/machine/peripheral-anon/device[0]",
  "property": "smart_critical_warning", "value":16 } }'
2, run smartctl in guest
 #smartctl -H -l error /dev/nvme0n1

  === START OF SMART DATA SECTION ===
  SMART overall-health self-assessment test result: FAILED!
  - volatile memory backup device has failed
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Nzhenwei pi <pizhenwei@bytedance.com>
Signed-off-by: NKlaus Jensen <k.jensen@samsung.com>