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

[Richard W.M. Jones]

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.