postinst is in nova-compute-libvirt 1:2014.1.5-0ubuntu1.7.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 | #!/bin/sh -e
if [ "$1" = "configure" ]; then
    if ! getent group libvirtd | grep -qE '\<nova\>'
    then
        adduser nova libvirtd
    fi
fi
 |