log: update docs for daemons to improve user understanding

Strongly recommend against use of the log_levels setting since it
creates overly verbose logs and has a serious performance impact.

Describe the log filter syntax better and mention use of shell
glob syntax. Also provide more realistic example of good settings
to use. The libvirtd example is biased towards QEMU, but when the
drivers split off each daemon can get its own more appropriate
example.

Reviewed-by: John Ferlan <jferlan@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>