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.