Run sanity checks before partitioning.

This should avoid errors such as if there is no "/" device specified or if
partitions are too small for some file systems and other stuff.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Print out nix configuration expression when done.

This now prints out fileSystems and swapDevices attributes, which can be used
for adding/importing it into configuration.nix.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>