I have a HP Zbook laptop running CentOS7.1. I use it to run an Ubuntu 14.04 VM, which has been working fine for quite a while.
This morning I saw some weird problems (can't remember exactly what), so I rebooted, and it wouldn't show the Gnome login page (I saw the line vector "7" in the background). I eventually figured out how to get the text login prompt, and it showed that a recent yum update of VirtualBox appeared to be aborted. I did "yum-complete-transaction" and that appeared to complete.
I don't know why that would block the graphical login, but when I rebooted after that, I got the login page back and everything seemed fine, until I tried to run my VM.
When I did that, I got a dialog saying this:
Failed to start the virtual machine ... Implementation of the USB 2.0 controller not found!
What can I do now?