Qos feature: Adds bandwidth definition at configuration level.

Network definition when configuring libvirt network is enriched with
the optional bandwidth parameters for traffic shaping. Patch includes
a refactoring of the xml generation and some tests.

Change-Id: If0c3b4b9a6fd5eb53579fb2e157b5325caa88d04
Signed-off-by: Giuseppe Vallarelli <gvallare@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/15724
Reviewed-by: Dan Kenigsberg <danken@redhat.com>