Since we can use the FieldSet class in cli.py to nicely format disk
sizes and such, we move it to utils.py and also move its associated
unittest. I didn't remove the cmdlib.py unittest file as that's not the
good direction :)

Reviewed-by: ultrotter

This patch adds a _FieldSet class that can be used for the new variable
parameter sets: e.g. the sda_size will change to disk/0.size (or
similar) and we need to both check validity and extract the index of the
parameter easily.

The patch also sorts the unittest list in Makefile.am.

Reviewed-by: ultrotter