Move existing softraid check into reusable function

Move existing softraid check into reusable function

No behaviour change, but get_softraid_chunks() reads much clearer.

The root disk is checked to be a softraid volume;  if so, devices for each
chunks are created to make installboot work later.

bioctl accepts "sd1" and "softraid0", the latter prints all volumes.
Output is identical if sd1 is the only volume.

OK afresh1