Apparently in latest VB releases the Guest Additions have to be built directly on the image.
This of course is a pain, because OOTB the Centos7 image doesn't contain the development tools. SO you get a message like "building the main Guest Additions module Failed"
https://unix.stackexchange.com/questions/37866/how-to-solve-building-the-main-guest-additions-module-failed
this fixed the problem:
sudo yum install gcc kernel-devel-$(uname -r)
This of course is a pain, because OOTB the Centos7 image doesn't contain the development tools. SO you get a message like "building the main Guest Additions module Failed"
https://unix.stackexchange.com/questions/37866/how-to-solve-building-the-main-guest-additions-module-failed
this fixed the problem:
sudo yum install gcc kernel-devel-$(uname -r)