Install VirtualBox Guest Aditions on CentOS 7 Linux Guest
If you want to be able to move your mouse cursor outside the VirtualBox CentOS Guest window without pressing a key and also automatically scale CentOS Guest window you need to install VirtualBox Guest Aditions on your CentOS Guest.
In order to do that run:
yum install dkms yum install kernel-devel
Then add the cd from VirtualBox (Devices -> Insert Guest Aditions CD image). If you are using a window manager (like for example Mate) the iso will be mounted in /run/media where we can find the install script:
cd /run/media/root/VBOXADDITIONS_5.0.18_106667/ ./VBoxLinuxAdditions.run