[Libguestfs] [PATCH libnbd] interop: Add test of qemu-storage-daemon.

This commit adds a simple test of qemu-storage-daemon (QSD). On the basis that QSD is just qemu-nbd in new clothes this is only a simple test, not complete coverage. Nor does it test the unique features of QSD like being able to use QMP to create new server instances. Unfortunately QSD is not yet stable upstream. This version works with qemu 5.1.0 but at least two of the command line parameters have changed name upstream (see comment in patch) so it will break soon. Also there are quite a few other problems with QSD: https://lists.gnu.org/archive/html/qemu-devel/2020-09/threads.html#10952 So this is probably not something we can take upstream in libnbd yet, but here's the patch to look at. (I could push a patch with the test disabled and plan to enable it later.) Rich.