subprocess_helpers: Fix passing input to the process.
guestfish: Fix typos and log commands.
Update README.md with guestfs instructions.
guestfish: Fail if it's launched with an unprivileged user.

guestfish doesn't work with unprivileged users for me, at least
not on Ubuntu 12.10 (must be tested under other systems as well).
qa_make_disk: Turn all generic exceptions into fatal calls.

It's much more cleaner to see a log with fatal error messages
instead of exception and stack traces.
ostbuildlog: Add an utility function to print errors.