I've an *.img file, that can't be converted to *.iso, because its a multi-session disk. So I simply changed the file ending to .iso.
Now, I am able to mount it using the following command: mount -o loop dvd.iso /media/iso
BUT: When I try to use this iso in VirtualBox, it fails! Is there a way to mount the iso file as a drive, and then use it in VirtualBox?