I was trying other commands through qemu-guest-agent and they worked fine.

$ virsh guestinfo focal
user.count          : 1
user.0.name         : ubuntu
user.0.login-time   : 1591075860941
...

$ virsh guestvcpus focal
vcpus          : 0
online         : 0
offlinable     : 

Also free form commands were fine and here we can also see what is enabled.

$ virsh qemu-agent-command focal '{"execute": "guest-info"}' | jq .
{
  "return": {
    "version": "4.2.0",
    "supported_commands": [
      {
        "enabled": true,
        "name": "guest-get-osinfo",
        "success-response": true
      },
      {
        "enabled": true,
        "name": "guest-get-timezone",
        "success-response": true
      },
      {
        "enabled": true,
        "name": "guest-get-users",
        "success-response": true
      },
      {
        "enabled": true,
        "name": "guest-get-host-name",
        "success-response": true
      },
      {
        "enabled": true,
        "name": "guest-exec",
        "success-response": true
      },
      {
        "enabled": true,
        "name": "guest-exec-status",
        "success-response": true
      },
      {
        "enabled": true,
        "name": "guest-get-memory-block-info",
        "success-response": true
      },
      {
        "enabled": true,
        "name": "guest-set-memory-blocks",
        "success-response": true
      },
      {
        "enabled": true,
        "name": "guest-get-memory-blocks",
        "success-response": true
      },
      {
        "enabled": true,
        "name": "guest-set-user-password",
        "success-response": true
      },
      {
        "enabled": true,
        "name": "guest-get-fsinfo",
        "success-response": true
      },
      {
        "enabled": true,
        "name": "guest-set-vcpus",
        "success-response": true
      },
      {
        "enabled": true,
        "name": "guest-get-vcpus",
        "success-response": true
      },
      {
        "enabled": true,
        "name": "guest-network-get-interfaces",
        "success-response": true
      },
      {
        "enabled": true,
        "name": "guest-suspend-hybrid",
        "success-response": false
      },
      {
        "enabled": true,
        "name": "guest-suspend-ram",
        "success-response": false
      },
      {
        "enabled": true,
        "name": "guest-suspend-disk",
        "success-response": false
      },
      {
        "enabled": true,
        "name": "guest-fstrim",
        "success-response": true
      },
      {
        "enabled": true,
        "name": "guest-fsfreeze-thaw",
        "success-response": true
      },
      {
        "enabled": true,
        "name": "guest-fsfreeze-freeze-list",
        "success-response": true
      },
      {
        "enabled": true,
        "name": "guest-fsfreeze-freeze",
        "success-response": true
      },
      {
        "enabled": true,
        "name": "guest-fsfreeze-status",
        "success-response": true
      },
      {
        "enabled": true,
        "name": "guest-file-flush",
        "success-response": true
      },
      {
        "enabled": true,
        "name": "guest-file-seek",
        "success-response": true
      },
      {
        "enabled": true,
        "name": "guest-file-write",
        "success-response": true
      },
      {
        "enabled": true,
        "name": "guest-file-read",
        "success-response": true
      },
      {
        "enabled": true,
        "name": "guest-file-close",
        "success-response": true
      },
      {
        "enabled": true,
        "name": "guest-file-open",
        "success-response": true
      },
      {
        "enabled": true,
        "name": "guest-shutdown",
        "success-response": false
      },
      {
        "enabled": true,
        "name": "guest-info",
        "success-response": true
      },
      {
        "enabled": true,
        "name": "guest-set-time",
        "success-response": true
      },
      {
        "enabled": true,
        "name": "guest-get-time",
        "success-response": true
      },
      {
        "enabled": true,
        "name": "guest-ping",
        "success-response": true
      },
      {
        "enabled": true,
        "name": "guest-sync",
        "success-response": true
      },
      {
        "enabled": true,
        "name": "guest-sync-delimited",
        "success-response": true
      }
    ]
  }
}