In order to stop or start virtual machines you can use either the virsh command or the virt-manager utility.
With the virt-manager utility you can select your virtual machine and then choose either the shutdown or force off option. To run virt-manager type the following in terminal.
You can also manage the virtual machine directly from terminal. In order to power on a virtual machine using virsh do the following.
virsh start domain_name
The above command will start the virtual machine. Make sure the replace the domain name with the appropriate domain.
To shutdown the system you have 2 options. You can use the shutdown option which will go through the process of shutting down the system properly. Keep in mind that this method does not always work to plan. So instead you can use the destroy option which will force shutdown the machine, similar to you just hitting the power button on the system.
virsh shutdown domain_name
virsh destroy domain_name