tappd: Implement vsock-based guest API

[kvinwang]

This PR adds vsock api implementation in tappd. Currently defined APIs are:

service GuestApi {
  rpc Info(google.protobuf.Empty) returns (GuestInfo);
  rpc SysInfo(google.protobuf.Empty) returns (SystemInfo);
  rpc NetworkInfo(google.protobuf.Empty) returns (NetworkInformation);
  rpc ListContainers(google.protobuf.Empty) returns (ListContainersResponse);
  rpc Shutdown(google.protobuf.Empty) returns (google.protobuf.Empty);
}

The APIs are also proxied by teepod for Cloud management.

This PR also includes graceful shutdown VMs.