add libvirtd logs to logstash

The gate surely runs into libvirt bugs on a regular basis, but we don't
capture those logs in logstash, so we can't write elastic-recheck
queries against them.

For example, I believe the underlying cause of this issue is libvirt:

  https://bugs.launchpad.net/tempest/+bug/1276778/comments/10

Change-Id: Ie2f03b19249967e78319a4016921f91ef0498540
Merge "add libvirtd logs to logstash"