Fix collectd decoder for Xenial compute nodes

Xenial ships collectd 5.5 which has renamed the libvirt plugin to virt.

Change-Id: I36b4f53a755b1498d5c89db55183df088ad6630f
diff --git a/heka/files/lua/decoders/collectd.lua b/heka/files/lua/decoders/collectd.lua
index 7978bd2..36daee0 100644
--- a/heka/files/lua/decoders/collectd.lua
+++ b/heka/files/lua/decoders/collectd.lua
@@ -457,7 +457,7 @@
                 msg['Fields']['name'] = 'logged_users'
                 msg['Fields']['hostname'] = sample['host']
                 table.insert(msg['Fields']['tag_fields'], 'hostname')
-            elseif metric_source ==  'libvirt' then
+            elseif metric_source ==  'libvirt' or metric_source == 'virt' then
                 -- collectd sends the instance's ID in the 'host' field
                 msg['Fields']['instance_id'] = sample['host']
                 table.insert(msg['Fields']['tag_fields'], 'instance_id')