What about "Disable ACPI:" in Options tab of the vm configuration in the Proxmox web interface?
You can flag it, go in the shell of proxmox, and issue:
# qm showcmd VMID
and you will see the parameters will be used to run the vm (and find -no-acpi too, in this case).
In any case, you can use:
args:
in the /etc/qemu-server/vmid.conf file to add whatever option you want, like:
args: -enable-nesting