Konverting from VirtualBox to KVM is actually quite easy.
If you use snapshots in VirtualBox, then you will first have to clone the current running state into a new VM (Right clickt -> Clone -> blah -> select only current state)
Then, if you use dynamic growing disks, convert them to raw format
VBoxManage clonehd --format RAW Win7.vdi Win7.raw
Now convert it to qcow2