[Libguestfs] [PATCH v2 nbdkit] tests: Add generic requires.

v1 was here: https://www.redhat.com/archives/libguestfs/2019-January/thread.html#00198 For v2 I changed most existing prerequisite tests to use the new mechanism. I only changed simple tests. There are a few more complex tests that don't fit the “requires model” and those are not changed. I normalized qemu-io/qemu-img testing to always use the --version flag, where previously we used a mix of --version and --help. The requires tests are generally done above the cleanup function, to avoid doing cleanup if we're not going to run the test. Error messages are sometimes changed, but nothing important. Tested it on Linux, FreeBSD and OpenBSD. Rich.