Bypass `remote_tunnel()` for vSphere (Bug Report)

I realized that the latest micro plugin does not work with vSphere. It stops on waiting for the agent.
It seems that the loop in `remote_tunnel()` never breaks because `IO.select` in `socket_readable?` always fails. This is caused by that some instance variables (e.g. @ssh_*) are missing for the subclass for vSphere.

I created an ad-hock patch for this issue. I'm not sure if SSH tunneling is required for vSphere too, so I simply added a overiding method to bypass it.