refactoring: Changing constant name in NICENESS enum in storage.misc module.

Within the storage.misc module there's an enum representing NICENESS
for processes, NICENESS.LOW becomes NICENESS.HIGH which represents
the lowest priority.

Change-Id: Idaeef1c660c8efc0e715d506be5b9ebf4063e8bd
Signed-off-by: Giuseppe Vallarelli <gvallare@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/14533
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>