netconfpersistence: add diffing method to Config

This patch makes it easy to diff two network configurations
in a way that gives you a resulting Config object that can
be used for a setupNetworks operation.

If the current configuration is B and we want to put the
system in state A, we'd do:

    diff = A.diffFrom(B)
    setupNetwork(diff.networks, diff.bonds, {})

Change-Id: I70a3be5610492fd3cb27d49703e04a84cfad57d1
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21728
Reviewed-by: Dan Kenigsberg <danken@redhat.com>