It's not even that hard, it just takes some time, a Ubuntu LiveCD, and an external USB disk (if you don't have more than one internal disks).
Preliminary step: Convert the disk to something useful
You can do this from your current system without having to boot to a LiveCD.
Load a terminal and fire in:
cd /media/wherever-the-image-is/
sudo apt-get install qemu-kvm
qemu-img convert test.qcow2 -O raw disk.img
Move /media/wherever-the-image-is/disk.img somewhere that you're not about to write to. If you're planning to write it to the disk that it's currently sitting on, you'll want to stick it on a separate internal disk, or, worst-comes-to-worst an external disk.
The following instructions assume you've moved it to /media/dave/disk.img (dave is an external USB disk)
Before you do any serious writing, make sure you have backups. CloneZilla can help you take whole disk backups if you have somewhere for that data to be stored.
Write the image to a disk of its very own
You'll want to do something like this. This assumes you're going to overwrite a whole disk.
Boot into an Ubuntu Live CD and click Try Ubuntu.
Mount your the place where your qcow2 image is being stored (eg the external USB disk as /media/dave). Do not mount the place where you want to write to.
Then we go to work:
sudo dd if=/media/dave/disk.img of=/dev/sdX
You want to replace sdX with the correct path to your destination disk. The sudo password is blank, just hit return.
You can then open gparted or something else and you should see your Ubuntu partition sitting on the disk. You should be able to expand it out.
As I understand it, Ubuntu has deprecated vmbuilder. Long-time vmbuilder users, like myself, are now forced to find other, supported methods for creating virtual machines - like virt-builder and virt-install from the libguestfs project. Canonical has abandoned both JeOS and vmbuilder as they move their focus onto non-server deployments (e.g. mobile). As much as it pains me to say it, this may be part of larger Canonical shifts away from server/cloud support (in perennial pursuit of a profitable business model).
Best Answer
If you do not already have KVM installed, you will need to do that. There is a
kvmmetapackage in the repos.To create VMs the easy way, use virt-manager (available in the repos). It provides a nice GUI to walk you through it.
Keep in mind you can use virt-manager either on your local KVM server, or on a remote one. Once you have it fired up and pointed to the right KVM server, just click the "Create New Virtual Machine" button. The rest of it is fairly self-explanatory.
Also, if you want to use a bridged interface for KVM, follow my guide here: http://john.wesorick.com/2012/01/setting-up-bridged-network-interface.html
Bridged mode works better for any kind of networking on the virtual machine.