Only happens in an obscure case. Since virtual nodes can be fixed to other
topology nodes in the NS file, "virthost" nodes can explicitly appear in the
resulting topology descriptions. However, virthost nodes do not run linktest
so we need to ensure they do not get nominated as "linktest master" for an
experiment.

Rather than fix this on the client-side and create further backward-compat
issues, we resolve this in the server-generated linktest topology map.
By marking such virthost nodes as not supporting linktest, the client side
will automatically weed out those nodes.