OpenStack requires a system that has the virtualisation feature, but you might want to test it on the virtual guest before actually install it in bare metal system.
In that case, you can achieve it by enabling nested virtualisation feature in recent Linux distribution. You can find if your distro has that feature by checking it with the following command.
$ modinfo kvm_intel | grep nested
If it does show you the parameter, it means your system has the feature. Otherwise, you might need to try it with the new version of distro. In my case, Fedora 17 has the feature, but not in RHEL5 or RHEL6 yet.
If the feature is there, you just need to create a file that enables the feature. In my case, I just created a file /etc/modprobe.d/kvm_intel.conf with the below content.
options kvm_intel nested=1
After reboot, you can confirm it the feature is enabled or not by running the below.
$ cat /sys/module/kvm_intel/parameters/nested Y