The following does not achieve system shutdown "send_command (system_powerdown)":
echo "system_powerdown" | socat -u - UNIX-CONNECT:/run/vm/kvm.${VMNAME}.monitor
echo "system_powerdown" | nc6 /run/vm/kvm.${VMNAME}.monitor --send-only

Removing switch -u (unidirectional mode) does cause socat to send a system shutdown
echo "system_powerdown" | socat - UNIX-CONNECT:/run/vm/kvm.funtoo.monitor

Adjusted /etc/init.d/qemu accordingly. Please see qemu-init-script.patch

ebuild:
Added sys-apps/usermode-utilities as a requirement to ebuild as tunctl is required to create tun devices.
Removed OR package requirement "|| ( net-misc/socat net-analyzer/netcat6 )"
In favor of only net-misc/socat

Adjusted ebuild. Please see qemu-init-scripts-0.3.0.ebuild.patch