How to change the keyboard layout?

Become root with su -. Then rerun keyboard configuration and restart the console service:

# dpkg-reconfigure -p high keyboard-configuration
# service console-setup restart

How to convert disk images between formats?

VirtualBox does not support disk images with backing files, so they must be combined into a single file before setting up the virtual machine. To convert disk images, download and install QEMU, which should give you the tool qemu-img (on Debian-derived distributions it is enough to apt install qemu-tools).

To convert a disk image disk.qcow2 to VDI, navigate to the directory with the downloaded files and run

$ qemu-img convert -O vdi -p disk.qcow2 disk.vdi

to join the image with all required backing files into a single disk image in the VDI format. On Windows, you will probably have to use the full path to call qemu-img:

> "C:\Program Files\qemu\qemu-img.exe" convert -O vdi -p disk.qcow2 disk.vdi

Resulting images can be used in VirtualBox as well as in GNS3 and other qemu-based programs.

What are the usernames and password for virtual machines?

On arbiter, login as test / test. On other virtual machines you can login as root / kaboom or as the regular user student / vaje.