Previously QEMU rejected IPv6 host-forward attempts that had an
unspecified guest address. This is because for IPv6 the guest's
IP address isn't necessarily known ahead of time: Libslirp only
provides a "stateless" DHCPv6 server, which if the macaddr is
random then the IPv6 address is random too.

This patch changes this to do the address resolution lazily, in the
hopes that the guest's IPv6 address is known at the time the user
wants to connect to the guest. The request can still fail if the
guest doesn't have an IPv6 address yet (e.g., it's still early in
the boot). Such requests are immediately rejected.

Signed-off-by: Doug Evans <dje@google.com>