Raise a proper SSH error on EHOSTUNREACH

When using providers different from Virtualbox or Vmware Fusion/Workstation, it is perfectly possible to have the guest VM behind a routed network. In that case, in the event of a boot failure or a network misconfiguration, ssh connection attempts will probably fail with `Errno::EHOSTUNREACH` (No route to host) exception. That exception is not currently rescued in the ssh communicator, while probably it should be.