Once the para-virtualized drivers are installed, you will be able to shutdown or reboot the guest programmatically from the host.

To do so, you can type one of the following commands on the host, replacing w2k3_32 with the name of your guest:

# virsh shutdown w2k3_32
# virsh reboot w2k3_32

The two commands will respectively shutdown and reboot the machine.

The same functionality is available from the virt-manager graphical interface.