Ignore the AlternateLBA field in the GPT headers.

We know where to look, and we'll look there regardless. We don't care where
the header creator thinks it should be.

Update tests to match.

Oh, and don't assume that I mean "/dev/FOO" if I just say "FOO". That's
really annoying.

Review URL: http://codereview.chromium.org/2606002
4 files changed