[Libguestfs] [PATCH nbdkit v2] tests: Rename and rework test-ipv4.sh so it tests

This is my second go at reworking the ‘test-ipv4.sh’ test. Instead of merely connecting to the server using socat, use ‘qemu-img info’ so that the test is slightly more realistic. Also clean up old PID and log files before starting the test. Previous version was posted here: https://www.redhat.com/archives/libguestfs/2018-January/msg00253.html Rich.