fileSD: getMountsList should be a private function

Renaming getMountsList with private prefix as the function is not
expected to be used outside of fileSD.

Change-Id: Ifadead89b1e46656c8ba90ba83f64fc558920a15
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31134
Reviewed-by: Dan Kenigsberg <danken@redhat.com>