configNet: allow delete/update of devices with no ifcfg

In Fedora 18, ifcfg files are missing by default. This patch assumes
that there are no custom MTU setting for a device with no ifcfg file.

This version of the patch owes a lot to Mark Wu's
http://gerrit.ovirt.org/11357 and to Toni who convinced me that it is
better to read the MTU directly from kernel.

Change-Id: Icb3a623ca3d3b560288cbe4141eea6bd060ac798
Bug-Url: https://bugzilla.redhat.com/906383
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/11621
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Tested-by: Mike Kolesnik <mkolesni@redhat.com>