Add a boot menu toggle to the System or Drives page for pc & q35 machines

[conath]

There is a Boot Menu available with the pc and q35 machines' BIOS that can be enabled with the QEMU parameter -boot menu=on (I have personally verified that this doesn't work with the Tianocore EFI BIOS).

I would like to be able to turn on this boot menu for my VMs in the "System" configuration. This way I don't need to worry about the boot order and can override it when I need to. However I feel like the boot menu should not be enabled all the time, as it slows down booting the VM when I don't want to override the boot order.

[osy]

However I feel like the boot menu should not be enabled all the time, as it slows down booting the VM when I don't want to override the boot order.

I think the 1s delay is perfectly fine considering we added a 1s delay to workaround the USB bug in EFI. Adding an option is more confusing because 1) the menu doesn't appear in EFI boot and 2) it still requires you to press Esc.