Move getMaxMtu to netinfo

For a while now, getMaxMtu has not used any part of configWriter,
but just netinfo information. Thus, it is only right that it should
be moved to its rightful module, i.e., netinfo.py.

Change-Id: Ib994deab0d38661508dd6cc4c890a340000617ac
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/14856
Tested-by: Giuseppe Vallarelli <gvallare@redhat.com>
Reviewed-by: Giuseppe Vallarelli <gvallare@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>