I've been beating my head against the desk for WEEKS now trying to get Qemu to work with any friggin distro set up by UNetbootin, with or without Gujin to help. No matter what I tried, the response was qemu: could not open disk image /dev/sdh
Well, after messing with ownership & permissions, I'm a lot closer -- and wanted to document the possible solution for the next person plagued by this issue. (Based on what I saw while searching the web, I'm nowhere near the first or the least-experienced.) UNetbootin has an annoying little quirk, it turns out: every file it handles becomes owned by root, and evidently key files aren't marked as executable when Qemu thinks they should be. So first you have to give yourself ownership of everything (sudo chown -R yourusername) and then full permissions (sudo chmod a=rwx drivelocation).
With any luck, tomorrow I'll manage to get it to do something more interesting, like boot something... Non-Ubuntu (and hopefully non-crashing non-slow-on-this-computer) distribution, here I come!