Be sure to use full path names if you don't want your disk images to end up in ~/.VirtualBox/HardDisks/

# Convert the VB disk vdi image to a raw image format.
VBoxManage clonehd --format RAW ubuntu.vdi ubuntu.img

We can now use the raw image at top of a LVM LV

sudo qemu-img convert /data/vms/seedbox/seedbox.img -O raw /dev/vms/seedbox

Or we can just convert the raw image to qcow2

qemu-img convert -f raw/data/vms/seedbox/seedbox.img -O qcow2 /data/vms/seedbox/seedbox.qcow

Finally, we test our old new VM with:

virt-install --hvm --name $server-name \
--ram 4096 --vcpus 4 --disk path=/dev/mapper/$server-name-root \
--import --graphics none --serial pty \
--console pty --network bridge=br0

If it is working you may delete the raw image and the .vdi image.

Sources: