[Libguestfs] [PATCH libnbd 0/2] Change qemu-nbd interop tests to use socket activation.

Now that we have implemented systemd socket activation, we can use this to run qemu-nbd in tests. The first patch leaves some dead code around (the -DSERVE_OVER_TCP=1 path). It's possible we might want to use this to test against a putative future NBD server that only supports TCP, but on the other hand maybe we should just remove it. Tests & valgrind still pass for me. Rich.