While setting up a test box I came across this issue. The shutdown process hangs and waits.

These steps worked for me http://askubuntu.com/a/123287

Edit the grub config

sudo nano /etc/default/grub

Add acpi=force to this line

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi=force"

Save and update the grub config.

sudo update-grub