netinfo: reduce netinfo instantiation to be closer to linear

During each setupNetworks operation there were several points that
could call for a full NetInfo instantiation. This patch reduces
the amount of points that do so by replacing instantiation with
very fast system polling of:

- ifaceUsers: which will now check link objects and networks only if
  necessary
- vlansForIface: which will just use link objects.

Bug-Url: https://bugzilla.redhat.com/1053114
Change-Id: I424e0ba9a99951432dea0365e12a02b7fb7a5bf7
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23577
Reviewed-by: Dan Kenigsberg <danken@redhat.com>