Enabling it by default because:
- This allows you to get the relevant logs right away when something
in the libvirt code path fails, without having to submit another
change and keep doing a 'recheck' to re-run the CI check/gate jobs
until you hit the bug.
- The libvirt log filters specified in the function
'configure_libvirt' are much more _selective_ and not a catch-all
debug option where you end up with the unhelpful situation of having
to find a "specific piece of hay in a haystack"[1].
FWIW, I always have it enabled in local test environments, and I don't
see the resulting libvirtd.log growing beyond a couple of MB for
three-four days of usage.
[1] http://lists.openstack.org/pipermail/openstack-dev/2014-January/024414.html
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