Gotta love a tool that will generate the xml and virsh shell magic - (define the machine, boot the vm from the iso, blah blah blah)...
and even create the disk image for ya! Just connect to the virtual instance with virt-viewer and you are off to the races.
Here is an example (bridged networking mind ya), with a 50G disk 1GB ram, and defaults for most other items:
virt-install --connect qemu:///system -n openfiler1 -r 1024 --vcpus=2 -f /usr/local/vms/openfiler1.qcow2 -s 50 -c /root/isos/openfiler-2.3-x86_64-disc1.iso --vnc --noautoconsole --os-type linux --network=bridge:br0 --hvm
To figure out if that is what you needed, you can use virt-xml-validate before defining the new xml with virsh.
The page here is where to go to add things yourself: http://libvirt.org/format.html
For even more fun, make a vm on the fly:
virt-install --prompt