It is easy to convert a VirtualBox VDI image to a KVM qcow2 file. You have to use the RAW file format as an intermediate.

Make sure the VirtualBox machine is shutdown.

1. Convert the VDI to a raw disk image. Note: VDIs are compressed and raw images are not, so you will need to leave enough disk space for entire uncompressed disk. $ VBoxManage clonehd –format RAW vm.vdi vm.img
2. Then on your KVM host: $ qemu-img convert -f raw vm.img -O qcow2 vm.qcow2