In den ersten Lebensjahren eines Kindes bringen ihm die Eltern Gehen und Sprechen bei,
in den späteren verlangen sie dann, dass es stillsitzt und den Mund hält.
(Johann Nestroy)

Under Windows, there has already been a driver for mounting Virtualbox VDIs under Windows, called ImDisk (which I have never tried). For Linux, there was a default method using losetup (which I also didnt try, because it sounded like these hacky solutions that often fail and break your whole virtual disk).

Anyway, meanwhile, Ubuntu has a package named virtualbox-ose-fuse, containing a tool called "vdfuse", which creates a fuse filesystem containing a file that can be loopmounted (the partitions are added, but I prefer kpartx).

The command is self-explaining and trivial to use, a little bit of configuration for fuse is necessary, but thats all. A pretty useful tool.