It seems as this isn't impossible but I haven't found any software that does this, except this one, but it isn't even released yet: vBoot.
My objective is:
- Install Windows XP in a VM and save it as an image.
- Put that image in a storage medium such as USB or disk.
- Boot an arbitrary machine directly on a VM that reads that image.
- (Optionally) save changes to the image before shutting down.
It's ok if I have to get used to new tools (up to date VirtualBox solved all my needs).
It's also ok if I have to deal with scripting, even some coding, or anything that can help me achieve that task, as it would be very useful to have a portable WinXP VM that I can boot into.
In a nutshell: to run VirtualBox/VMWare/other XP image without booting into a preinstalled OS.