On qemu docs, pipe image directly to bzcat

Saves ~200MB of disk space, and one command-line instruction :)
(We could use `curl` to avoid the `-O -` parameter to `wget`, but I think it's a bit weird to use `wget` in one line and then `curl` on the next one)
Merge pull request #88 from riccieri/patch-1

On qemu docs, pipe image directly to bzcat