Did you run this through gofmt?  There is no need to break lines in Go if the line length is reasonable.
Alternatively, another way to break this up is:
```go
s := fmt.Sprintf("hostonly_docker,tcp,%v,%v,,%v", B2D.HostIP, B2D.DockerPort, B2D.DockerPort)
if err := vbm("modifyvm", B2D.VM, "--natpf1", s); err != nil {
//...
```
(I prefer using `%v` almost everywhere, except maybe in tests where `%q` is needed... but you can change back to `%s` and `%d` if you want since this project tends to do that a lot anyway.)