gtools: Fix collectd plugin wrt netstats

Ganeti with ifdown support adds a new file per NIC (with
name it's UUID) under /var/run/ganeti/kvm-hypervisor/nic/<vm>/.
Let ganeti-netstats.py collectd plugin be aware of this change.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>