[libvirt] IPv6 migration

Hello. We can only tell QEMU on the destination to listen either on IPv6 or on IPv4. If we're supplied with a numeric v6 address, that's the only thing we need to know to set the listen address to [::]. For hostnames, we can either assume this based on how it resolves by default on the destination (we keep trying all the resolved addresses on the source, but this might break a few cases), which John found disgusting, so that leaves user input: How about a VIR_DOMAIN_MIGRATE_IPV6 flag, depending on which we set the listen address on the destination and creating a new function virNetSocketNewConnectTCPHints, where we would add IPv4/IPv6 hint based on the presence/absence of this flag? Would it be better to auto-add this flag for numeric addresses, or just check for them both in the Prepare phase on the dest (and set listen_addr accordingly) and before connecting from the source, so we don't use the IPv4 hint (in case we wanted to use the partial fix for just numeric addresses for older libvirt). Jan Bug: https://bugzilla.redhat.com/show_bug.cgi?id=846013 v3: https://www.redhat.com/archives/libvir-list/2013-February/msg01379.html unfortunately I posted v3 before reading John's feedback to v1: https://www.redhat.com/archives/libvir-list/2013-February/msg01059.html