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>