Mounting a VDI file with qemu-nbd

$ sudo modprobe nbd
$ qemu-nbd -c /dev/nbd0 [vdi-file]

Example:

Connect VDI to loopback device /dev/nbd0:

$ qemu-nbd -c /dev/nbd0 /home/test/ubuntu.vdi

You can check the partitions in the image using the following command:

$ sudo fdisk -l /dev/nbd0

If you want to mount the first partition of the NBD device, use the following command:

$ sudo mount /dev/nbd0p1 /mnt

Finally, unmount /mnt then shutdown nbd services using the following command:

$ sudo qemu-nbd -d /dev/nbd0

It is advised to add --read-only before the first command if not editing a file, as a security step.

Converting a VDI file into a partition

Use qemu-img:

$ sudo qemu-img convert -m J -p -f vdi [vdi-file] /dev/sdX

where J is the number of threads allowed.