[Libguestfs] [PATCH nbdkit] tests: Convert some tests to use nbdsh instead of qemu-io.

Very much a work in progress as there are still many tests using qemu-io which are candidates for conversion. You'll notice at the end of test-full.sh that the new test has some duplicated code which looks as if it ought to be refactored into a Python function. When I tried to do that, I got loads of strange Python problems which may indicate bugs in nbdsh itself or problems with my understanding of how Python works, but I wasn't sure how to fix it. The current implementation works. Rich.