The following error was encountered while trying to get libvirtd running:
libvirt version: 0.9.10, package: 21.el6_3.1 (CentOS BuildSystem , 2012-07-03-16:15:49, c6b8.bsys.dev.centos.org)
error : virNetServerMDNSStart:460 : internal error Failed to create mDNS client: Daemon not running
Note that I had to check /var/log/libvirt/libvirt.log as a service libvirtd start looked fine, a restarted was failing on stopping it.
The issue comes from avahi not running. Go ahead and install it and get messagebus running via:
yum install avahi -y
service /messagebus restart
service avahi-daemon restart
service libvirtd restart
chkconfig --level 345 avahi-daemon on
chkconfig --level 345 messagebus on
That should get you back up and running as well as making sure they come up next time you reboot the machine.