Prior to this change, each ceph cluster node would end up with several
"qemu-client-$pid.log" files owned by root. The [client] section would
capture *all* client activity (for example the "ceph health" command,
etc), not just librbd-in-qemu.

Restrict this section to libvirt clients only so that we don't generate
these spurious log files for other Ceph client traffic.

Signed-off-by: Ken Dreyer <kdreyer@redhat.com>