在virtualbox安装目录找到VBoxGuestAdditions.iso复制到虚拟机里面，可以放到任意一个目录
sudo mkdir /media/GuestAdditionsISO
sudo mount -o loop VBoxGuestAdditions.iso所在目录/VBoxGuestAdditions.iso /media/GuestAdditionsISO
这时候你会看到一条信息，the ISO has been mounted as READ-ONLY
cd /media/GuestAdditionsISO
./VBoxLinuxAdditions.run