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

[Richard W.M. Jones]

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.