Separate adding libvirt network from configuring host network.

It makes it simpler to test the network configure functions, and
also make the code a little bit cleaner.

Change-Id: I0bb352ac7c298baa4a3e4a5cba7d23b0b033952f
Signed-off-by: Mark Wu <wudxw@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/14880
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Tested-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>