It is very easy to convert a VirtualBox .vdi disk images to KVM qcow2 file. You have to use the RAW file format as an intermediate.
1 – Make sure the VirtualBox machine is shutdown.
2 – Convert the vdi to a raw disk images. Note: vdi are compressed and raw images are not and so you will need to leave enough disk space for entire uncompressed disk.
VBoxManage clonehd --format RAW vm.vdi vm.img
3 – Then on your KVM host:
qemu-img convert -f raw vm.img -O qcow2 vm.qcow2
Hope this is helpful.