Toying with docker, I found myself with an upgraded version of Virtual Box, and a new virtual machine brought me to the old issue of the pre-configuration needed for installing guest additions on CentOS 7. Things have become simpler:
1) The Extra Packages for Enterprise Linux repository is now better embedded, and therefore the following suffices to define it:
yum install epel-release
2) Having the new repositry, we can easily install dkms:
yum install dkms
And surprisingly, this solves all dependencies required for the guest additions... (at least it solved it for me, in a base+X installation).
so afterwards, we can run the guest additions installer, and see it complete.