util_linux: Add AppendToFileAsRoot utility function

Add a utility function similar to WriteToFileAsRoot(), that appends to the
specified path instead of overwriting the file. We use this in a following
patch to append to Qemu bridge access configuration file.

pkg/os/util_linux.go

@@ -10,8 +10,22 @@ import (
 )
 
 func WriteToFileAsRoot(reason, content, filepath string) error {
+	return writeToFileAsRoot(reason, content, filepath, false)
+}
+
+func AppendToFileAsRoot(reason, content, filepath string) error {
+	return writeToFileAsRoot(reason, content, filepath, true)
+}
+
+func writeToFileAsRoot(reason, content, filepath string, append bool) error {
 	logging.Infof("Will use root access: %s", reason)
-	cmd := exec.Command("sudo", "tee", filepath) // #nosec G204
+	var append_option string
+	if append {
+		append_option = "-a"
+	} else {
+		append_option = ""
+	}
+	cmd := exec.Command("sudo", "tee", append_option, filepath) // #nosec G204
 	cmd.Stdin = strings.NewReader(content)
 	buf := new(bytes.Buffer)
 	cmd.Stderr = buf