This installs power off handler that access power management register on PIIX4.
To use
.
cd /tmp 
tar -zxf intpmpm.tar.gz
cd $SYS/pci
patch </tmp/intpmpm/intpmpci.diff
cd $SYS/conf
patch </tmp/intpmpm/options.diff
cd $SYS/i386/conf
patch </tmp/intpmpm/LINT.diff
and add

controller intpm0
controller smbus0
options	PIIX_POWEROFF_ENABLE

to your configuration and reconfig and install kernel then reboot .