Optimize netinfo.networks.

Currently netinfo.networks is doing three trips to libvirt:
 - libvirtmod.virNetworkLookupByName
 - libvirtmod.virConnectListNetworks
 - libvirtmod.virNetworkGetXMLDesc

With this change, we reduce it to:
 - libvirtmod.virNetworkGetXMLDesc
 - libvirtmod.virConnectListAllNetworks

Testing
========

In [185]: %timeit foo()
100 loops, best of 3: 4.45 ms per loop

%timeit netinfo.networks()
%100 loops, best of 3: 7.55 ms per loop

Change-Id: I2cd332819363c4399335358686e19d6a1dc3b45e
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/13159
Reviewed-by: Dan Kenigsberg <danken@redhat.com>