Thanks to help of this mailing list (especially Jim Fehlig), I have finished setup of libvirt.
However, there is something weird. It is that I installed libvirt on 5 servers but 
the default connection of one of them is 'qemu:///' even if those of the others are all 'xen:///'.

I searched how to change it but, I only found a bypass adding " LIBVIRT_DEFAULT_URI='xen:///' " in my .bashrc file.
Currently, value of the variable (LIBVIRT_DEFAULT_URI) is set to 'qemu:///'.
Since the bypass makes the virtualization type of the server visible 'qemu' to the open stack, I have to change libvirt configuration.

I also checked some configure files under '/etc/libvirt', such as libvirt.conf, libvirtd.conf and libxl.conf.
But I have no idea, how and what to change.
Please give me some advise. It will be a big help for me.