Virtual machine creation and guest operating system installation are two different tasks. Creating a VM is like provisioning new PC hardware, but you need to install the OS separately
As you have seen with virt-manager or virt-install, a VM is first configured with the desired hardware resources, then you use one of the support installation methods to install the OS. Installing the actual operating system (also known as the Guest in virtualization terminology) is done in exactly same manner as on a physical system; the operating system's installer asks for configuration details and configures the system accordingly.
What if both of these tasks are combined and a virtual machine is created (along with a full operating system installation) in one go? It would clearly help to deploy the virtual machines much more rapidly and in a more automated way.
Tools such as virt-builder and oz can be used to combine these two tasks and accelerate the installation of new...