I am running proxmox on my own homelab server, so I don't care that much about downtime, I can use stop mode to backup my vms without any problem. So, to backup and restore vm on another machine, just follow these steps:

1. stop vm
2. vzdump $vmid [--dumpdir /mnt/backup] # default dumpdir is /var/lib/vz/dump
3. qmrestore $path-to-dumped-vm $new-vmid

I transfered an OpenWrt vm to another proxmox node using these steps, works perfectly. I will write another blog about how to use OpenWrt and bridge network to set up private network in proxmox. And a big trap in Ubuntu cloud images.

For more about backup and restore proxmox vms, refer to official docs