Extract helper functions for updating nic and bonding's mtu

Change-Id: Ic82b1d9a27492619548f1c7ba3fced79ee68a1a8
Signed-off-by: Mark Wu <wudxw@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/14871
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Tested-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>