virt-manager: ungrab mouse pointer when machine goes idle, so screen saver can kick in

Issue

• When the mouse is captured by the active virtual machine window, the system screen lock does not activate even after the idle delay has elapsed. However, after the mouse is moved out of the virtual machine windows, the screen lock activates immediately and locks the screen.
• The gnome-screensaver does not activate with the active virt-viewer window.
• If VNC has the pointer grabbed, the host screensaver won't kick in no matter how long we wait.