I had to remove libvirt manually using kill. Now when I'm trying to start libvirtd it says
error: Unable to initialize network sockets. Check /var/log/messages or run without --daemon for more info.
I tried running /usr/sbin/libvirtd and it works fine except that there is no network.
I had some VMs running while killing the libvirt which was launched before the installation of opennebula. Later I noticed that opennebula had changed the user and group of qemu.conf to oneadmin. So I suspect that some of the remains of previous VM still exist.