Unify mtu internal representations as integers

In netmodels, configNetwork and netinfo.getMaxMtu we always handle
MTUs as being of integer type. There were only two exceptions to
this rule, in netinfo reporting code. This code makes getMtu
to return an integer just like getMaxMtu and makes the exceptions
be exceptional cases that will use str.

Change-Id: I461d944fbbc8208aea17fb301752e5e7192cf7c2
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/15396
Reviewed-by: Giuseppe Vallarelli <gvallare@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>