VBoxLinuxAdditions.run を実行して Unit vboxadd.service not loaded. になった時の対応方法

[MacのVirtualBoxに作った仮想マシンへGuest Additionsをインストールする](http://ponsuke-tarou.hatenablog.com/entry/2017/07/23/175358) をしていた時のしくじりです。

**環境**

- ホストOS : macOS Sierra Virsion 10.12.5
- VirtualBox : Version 5.1.22 r115126 (Qt5.6.2)
    - ゲストOS : CentOS Linux 7.3.1611(Core)
    - 接続 : ブリッジ接続
    - Yum version : 3.4.3


### 事象 : VBoxLinuxAdditions.run を実行したらエラーになってログファイルに`Unit vboxadd.service not loaded.`と書かれていた
```shell-session
$ sudo sh /mnt/cdrom/VBoxLinuxAdditions.run
Verifying archive integrity... All good.
Uncompressing VirtualBox 5.1.22 Guest Additions for Linux...........
VirtualBox Guest Additions installer
You appear to have a version of the VirtualBox Guest Additions
on your system which was installed from a different source or using a
different type of installer.  If you installed it from a package from your
Linux distribution or if it is a default part of the system then we strongly
recommend that you cancel this installation and remove it properly before
installing this version.  If this is simply an older or a damaged
installation you may safely proceed.

Do you wish to continue anyway? [yes or no] y
Copying additional installer modules ...
Installing additional modules ...
vboxadd.sh: Starting the VirtualBox Guest Additions.
Failed to set up service vboxadd, please check the log file
/var/log/VBoxGuestAdditions.log for details.

$ vi /var/log/VBoxGuestAdditions.log
▼▼▼▼▼▼▼▼▼▼▼▼ VBoxGuestAdditions.logの中身 ▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼
Redirecting to /bin/systemctl stop  vboxadd-x11.service
Failed to stop vboxadd-x11.service: Unit vboxadd-x11.service not loaded.
Redirecting to /bin/systemctl stop  vboxvfs.service
Failed to stop vboxvfs.service: Unit vboxvfs.service not loaded.
Redirecting to /bin/systemctl stop  vboxadd-timesync.service
Failed to stop vboxadd-timesync.service: Unit vboxadd-timesync.service not loaded.
Redirecting to /bin/systemctl stop  vboxadd-service.service
Failed to stop vboxadd-service.service: Unit vboxadd-service.service not loaded.
Redirecting to /bin/systemctl stop  vboxadd.service
Failed to stop vboxadd.service: Unit vboxadd.service not loaded.
vboxadd.sh: failed: Configuration file /var/lib/VBoxGuestAdditions/config not found.
vboxadd.sh: failed: Look at /var/log/vboxadd-install.log to find out what went wrong.
vboxadd.sh: failed: Look at /var/log/vboxadd-install.log to find out what went wrong.
vboxadd.sh: failed: modprobe vboxguest failed.
```

### 原因 : よくわからない
### 対応 : 勧められたとおりに一旦削除してやり直す

> we strongly recommend that you cancel this installation and remove it properly before installing this version.
> ざっくり和訳 :
> このバージョンをインストールする前に、このインストールをキャンセルして正しく削除することを強くお勧めします。

1. CentOSを停止します。

```shell-session
$ shutdown -h now
==== AUTHENTICATING FOR org.freedesktop.login1.power-off ===
Authentication is required for powering off the system.
Authenticating as: mana
Password: 
==== AUTHENTICATION COMPLETE ===
Connection to 192.168.0.6 closed by remote host.
Connection to 192.168.0.6 closed.
```
２. 削除する
![Screen Shot 2017-07-18 at 0.01.05.png](https://qiita-image-store.s3.amazonaws.com/0/159761/593f0f21-7dc1-a81f-431d-dcf33abb74ef.png)
![Screen Shot 2017-07-18 at 0.01.19.png](https://qiita-image-store.s3.amazonaws.com/0/159761/ad482ba2-9ad2-ad56-f94d-bed0a287aee8.png)

３. CentOSを起動して[Devices] > [Insert Guest Additions CD image...]を選択します。
４. マウントしてVBoxLinuxAdditions.run を実行します。

```shell-session
$ sudo mount /dev/cdrom /mnt/cdrom/
[sudo] password for mana: 
mount: /dev/sr0 is write-protected, mounting read-only
$ sudo sh /mnt/cdrom/VBoxLinuxAdditions.run
```