To install it, disable libvirtd sysv initscript:
    chkconfig libvirtd off
    service libvirtd stop

and enable libvirtd upstart job:
    cp  /usr/share/doc/libvirt-*/libvirtd.upstart \
        /etc/init/libvirtd.conf
    initctl reload-configuration
    initctl start libvirtd

Test:
    initctl status libvirtd
libvirtd start/running, process 3929
    killall -9 libvirtd
    initctl status libvirtd
libvirtd start/running, process 4047

I looked into the possibility to use the upstart script from Ubuntu or
at least getting inspiration from it but that's not possible. "expect
daemon" is a nice thing but it only works if the process is defined with
exec stanza instead of script ... no script. Unfortunately, with exec
stanza environment variables can only be set within upstart script
(i.e., configuration in /etc/sysconfig/libvirtd can't work). Hence, we
need to use script stanza, source sysconfig, and execute libvirtd
without --daemon. For similar reasons we can't use limit stanza and need
to handle DAEMON_COREFILE_LIMIT in job's script.