This should work for all VMs of the cluster:
for vmid in $(cat /etc/pve/.vmlist | grep '"type": "qemu"' | cut -d '"' -f2); do qm set ${vmid} --onboot 0; done
and this for LXCs:
for vmid in $(cat /etc/pve/.vmlist | grep '"type": "lxc"' | cut -d '"' -f2); do pct set ${vmid} --onboot 0; done