凄い悩んだのでマトメ。
味噌は kernel-devel をインストールしておくことでした。

# rpm --import http://dl.fedoraproject.org/pub/epel/RPM-GPG-KEY-EPEL-6
# yum -y install http://dl.fedoraproject.org/pub/epel/6/x86_64/epel-release-6-8.noarch.rpm

# cd /etc/yum.repos.d/
# wget http://download.virtualbox.org/virtualbox/rpm/rhel/virtualbox.repo
# yum update

# yum -y install kernel-devel
# yum -y install gcc
# export KERN_DIR=/usr/src/kernels/3.10.0-123.20.1.el7.x86_64/

# yum -y install VirtualBox-4.3
# /etc/init.d/vboxdrv setup
Stopping VirtualBox kernel modules                         [  OK  ]
Recompiling VirtualBox kernel modules                      [  OK  ]
Starting VirtualBox kernel modules                         [  OK  ]

※できたっっっつ!

参考
http://qiita.com/it__ssei/items/18d54746be2dac56988f#2-13
http://tecadmin.net/install-oracle-virtualbox-on-centos-redhat-and-fedora/