sockets: ensure we don't accept IPv4 clients when IPv4 is disabled

Currently if you disable listening on IPv4 addresses, via the
CLI flag ipv4=off, we still mistakenly accept IPv4 clients via
the IPv6 listener socket due to IPV6_V6ONLY flag being unset.

We must ensure IPV6_V6ONLY is always set if ipv4=off

This fixes the following scenarios

  -incoming tcp::9000,ipv6=on
  -incoming tcp:[::]:9000,ipv6=on
  -chardev socket,id=cdev0,host=,port=9000,server,nowait,ipv4=off
  -chardev socket,id=cdev0,host=,port=9000,server,nowait,ipv6=on
  -chardev socket,id=cdev0,host=::,port=9000,server,nowait,ipv4=off
  -chardev socket,id=cdev0,host=::,port=9000,server,nowait,ipv6=on

which all mistakenly accepted IPv4 clients

Acked-by: Gerd Hoffmann <kraxel@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>