To convert a QEMU qcow format disk to VirtualBox vdi format you need to have Qemu installed. Qemu has tools to help convert disk formats.

grant@workstation:~$ qemu-img convert hda-qcow.img -O raw hda.img
grant@workstation:~$ VBoxManage convertdd hda.img hda.vdi
VirtualBox Command Line Management Interface Version 1.6.0
(C) 2005-2008 Sun Microsystems, Inc.
All rights reserved.

Converting VDI: from DD image file="hda.img" to file="hda.vdi"...
Creating fixed image with size 1024966656 bytes (978MB)...

This will convert a QEMU qcow format disk image to VirtualBox vdi format.