support different network topologies in vmtest

* vmtest currently only supports a single vm with a host only network using hardcoded IPv6 addresses
* VMs should be able to connect to services running on the host or network with other VMs
  * There is some support for on-host services introduced in https://github.com/facebookincubator/antlir/commit/b99ddf94d507977b8eaae511e7fb0484c457ef1e, but it is very rudimentary

