It is at this point that I remember that I had hibernated Debian instaed of shutting it down, and then I moved the filesystem out from under it. So I reboot again, it works. First thing I do after login is check "df"... and the drive reports 5GBs used.
I'm not a master at kernel internals but I have at least part of an inkling that the missing 14 gigabytes of space had something to do with hibernating and restoring the machine multiple times.
In other news it looks like there is a branch vmbuilder-gui, which I'll have to look into sometime.