For those who are interested to be able to power off DSM 6.1.x VM via ACPI and get the shutdown working through Proxmox Virtual Environment,
I have recompiled the button.ko and evdev.ko from the poweroff package of this post in order to get this working on dsm 6.1.x/DS3615xs.
power-off_dsm61x_ds3615xs_x86.tar.gz