Generators for disks

This patch adds generators for Disk instances to the Haskell
test code. It uses somewhat more reasonable generators to
fill the fields instead of just arbitrary values.
'genDiskWithChildren' is a generator that generates a disk
with a specified number of disk children. To avoid shooting
ourselves in the foot we do not generate further (grand)
child disks for the child disks. 'genDisk' calls
'genDiskWithChildren' by requesting three children as a
resonable default.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>