[Libguestfs] [PATCH libnbd] examples: Fix theoretical cookie race in example.

Previously discussed here: https://www.redhat.com/archives/libguestfs/2019-July/msg00213.html It turns out that deferring callbacks is a PITA. (It would be a bit easier if C has closures.) However by rewriting the example we can avoid the need to use the cookie at all and make it run a bit more efficiently, so let's do that instead. Rich.