[Libguestfs] [PATCH nbdkit] tests: Rename and rework test-ipv4.sh so it tests IPv6

I wanted to change this test so it tries connections on both IPv4 & IPv6. Having it connect on both is the easy bit. Harder was making it not fail on machines that don't have IPv6 stack (or IPv4 in some rare cases). TBH I wasn't able to test this, but it seems like this should work. In the end we want to modify nbdkit so it can listen on only certain interfaces, but that's a bit more complicated. Rich.