net_scale: Allow NetInfo update just the devices

When deleting networks there was a lot of NetInfo instantiation
due to the need for checking the devices in each delNetwork
operation. That need for updated information does not include
network changes, so this change adds a method for just updating
the device information and reusing the netinfo objects.

Bug-Url: https://bugzilla.redhat.com/1053114
Change-Id: Ia32b0333052c05c92475bddcc5e9ce805fb82c94
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23575
Reviewed-by: Dan Kenigsberg <danken@redhat.com>