Allow the user to disable overriding the fileSystems option with
virtualisation.fileSystems by setting
`virtualisation.fileSystems = lib.mkForce { };`.
With this change you can use the qemu-vm module to boot from an external
image that was not produced by the qemu-vm module itself. The user can
now re-use the modularly set fileSystems option instead of having to
reproduce it in virtualisation.fileSystems.
