Call "diskimage f args ..." rather than "diskimage_f args ...".

Call "diskimage f args ..." rather than "diskimage_f args ...".

This means that no changes are needed in the main autocluster script
if new functions are added.

Unfortunately, this is an API change, so any add-ons to autocluster
that use the previous mechanism will need to be updated.  This is
pretty easy - just do this to your external file:

  1,$s/diskimage_/diskimage /g

Signed-off-by: Martin Schwenke <martin@meltin.net>