If you know that your computer will have shutdown issues, one possible solution is to add "acpi=force"
There is also a kernel default parameter set in VL: CONFIG_APM_REAL_MODE_POWER_OFF
This is supposed to work around some buggy bioses and it is enabled as default in VL. However, I found the default setting *prevented* full shutdown on my hardware. A shutdown would turn off the hard drive motor, then sit without powering down the machine. Recompiling with this set to 'off' allowed full power down.