Split up WissKI Instance code

internal/component/instances/wisski_exec.go

@@ -0,0 +1,8 @@
+package instances
+
+import "github.com/tkw1536/goprogram/stream"
+
+// Shell executes a shell command inside the instance.
+func (wisski WissKI) Shell(io stream.IOStream, argv ...string) (int, error) {
+	return wisski.Barrel().Exec(io, "barrel", "/bin/sh", append([]string{"/user_shell.sh"}, argv...)...)
+}