Fix Internal Server Error when vm's power status cannot be confirmed

[ysksuzuki]

This PR fixes the internal server error that aborts node list command when vm's powers status cannot be confirmed. pmctl2 node list lists VM list without showing internal server error. pmctl2 node show shows power_Status: Unknown as follows.

$ pmctl2 node show boot-0 | jq .
{
  "name": "boot-0",
  "taps": {
    "r0-node1": "pm_tap04562b3f",
    "r0-node2": "pm_tap61b6b82f"
  },
  "volumes": [
    "root",
    "seed",
    "sabakan"
  ],
  "cpu": 8,
  "memory": "4G",
  "uefi": false,
  "tpm": false,
  "smbios": {
    "manufacturer": "",
    "product": "",
    "serial": "fb8f2417d0b4db30050719c31ce02a2e8141bbd8"
  },
  "power_status": "Unknown",
  "socket_path": "/tmp/boot-0.socket"
}

[masa213f]

LGTM