When in a virtual machine window, some key combinations, such as Ctrl+Alt+F1, are recognized by the virtual machine host and are not passed to the virtual machine. To bypass the virtual machine host, Virtual Machine Manager provides sticky key functionality. Pressing Ctrl, Alt, or Shift three times makes the key sticky, then you can press the remaining keys to pass the combination to the virtual machine.
For example, to pass Ctrl+Alt+F2 to a Linux virtual machine, press Ctrl three times, then press Alt+F2. You could also press Alt three times, then press Ctrl+F2.
The sticky key functionality is available in Virtual Machine Manager during and after installing a virtual machine.