If IPv6 gateway address provided by dom0 isn't a link local address, add
a /128 route to it. Also, add this address on backend interfaces (vif*).
This is to allow proper ICMP host unreachable packets forwarding - if
gateway (address on vif* interface) have only fe80: address, it will be
used as a source for ICMP reply. It will be properly delivered to the VM
directly connected there (for example from sys-net to sys-firewall), but
because of being link-local address, it will not be forwarded any
further.
This results timeouts if host doesn't have IPv6 connectivity.
c281d6454f