refactoring: getMaxMtu updated implementation using max builtin.

Improvement to getMaxMtu implementation which now is more
succint by using max builtin function.

Change-Id: I2e3beef38675b2f74f2945247b8af1de3eeebc90
Signed-off-by: Giuseppe Vallarelli <gvallare@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/14743
Reviewed-by: Martin Sivák <msivak@redhat.com>
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>