I can not answer the question about vmbuilder, because I have never used it.
I would download the iso file for whatever Ubutnu 16.04 you want to use and then do, for example, this, from the same directory as the iso file:
sudo virt-install -n desk_dev -r 8192 --disk path=/media/newhd/desk_dev.img,bus=virtio,size=50 -c xenial-desktop-amd64-20160320.iso --network bridge=br0,model=virtio,mac=52:54:00:87:d2:f4 --video=vmvga --graphics vnc,listen=0.0.0.0 --noautoconsole -v --vcpus=4
Where I am naming the VM desk-dev, and giving it 8 gigabytes of memory, and putting the image file at /media/newhd/desk_dev.img with a size of 50 gigabytes, and use whatever I named the iso file as, in this case xenial-desktop-amd64-20160320.iso, with a bridged network. I assign the MAC address because I already have entries in my DHCP and DNS servers for such a VM. I use the vmvga driver because the default one sucks. I use VNC to communicate with the VM, you might want different. I give it 4 CPUs.