Die Statistik ist für den Politiker das, was für den Betrunkenen die Laterne ist.
Sie dient weniger der Erleuchtung als der Aufrechterhaltung des eigenen Standpunktes.
(Roland Koch)

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.