https://askubuntu.com/questions/19430/mount-a-virtualbox-drive-image-vdi/50290#50290 -by Maxime R.

Use qemu-nbd, the process is explained on serverfault and in this blog.

Basically, you'll have to install qemu if needed:

sudo apt-get install qemu

Then you'll need to load the network block device module:

sudo rmmod nbd
sudo modprobe nbd max_part=16

Attach the .vdi image to one of the nbd you just created:

sudo qemu-nbd -c /dev/nbd0 drive.vdi

Now you will get a /dev/nbd0 block device, along with several /dev/nbd0p* partition device nodes.

sudo mount /dev/nbd0p1 /mnt

Once you are done, unmount everything and disconnect the device:

sudo qemu-nbd -d /dev/nbd0