Installing VirtualBox Guest Additions 4.3.26 on CentOS 7 minimal install

#login as root or "su" there

mkdir /media/cdrom
mount /dev/cdrom /media/cdrom

# will be mounted as read-only, no worries

cd /media/cdrom
./VBoxLinuxAdditions.run

# it will skip all X installs since there is no X in minimal edition