VDI image support in QEMU

[Even Rouault]

Here's a copy of a message sent to QEMU devel list.

This patch adds support for reading, writing and creation (and thus
conversion) of VDI images (VirtualBox image format). Creation of derived
images above read-only base images also works, through a hack (the base image
filename is stored as the comment of the derived image)
It's mainly a thin wrapper around VirtualBox OSE source code.
It may not compile on non-Linux hosts, and it definitely won't work on big
endian hosts. So, there's still a bit of work to do for those interested by
running it on those platforms.