libvirtd.log: time to be less verbose

After several years, we can drop log verbosity to libvirt's default.
Currently, libvirt logs are filling up ovirt-nodes that run dozens of
VMs.

Change-Id: I251c2534d3284aa95bffa56143c449d5de887008
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1125237
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31135
Tested-by: mooli tayer <mtayer@redhat.com>
Reviewed-by: Francesco Romani <fromani@redhat.com>