Last week I had to use an existing virtual machine on my local computer, which is a mac. The origin host machine was running Linux with KVM for visualization. The disk image format is qcow2, which is the default format in QEMU and not supported by VirtualBox. But the qemu-img tool can convert a qcow2 image to VirtualBox’s vdi format.
# qemu-img convert -O vdi vda.img vda.vdi
After converting the format, the new disk image, named “vda.vdi”, can be used with VirtualBox.