Add new helper functions that will return the necessary stages to
prepare a disk image, i.e. take care of creating the raw image
file, partitioning it, creating the devices on it (LUKS2, LVM2),
and formatting all the file systems: `GenImageStagesPrepare`.
Additionally, some partition layouts require some post-processing,
e.g. LVM2 where the volume group needs to be renamed "offline".
For this the `GenImageFinishStages` was added.
Co-Authored-By: Achilleas Koutsou <achilleas@koutsou.net>
