Got it !
First I had a look at the openSUSE site :http://en.opensuse.org/VirtualBox_installation
and found out that I did not have the xalan-c and xerces-c packages installed.
Once this was OK I reinstalled VirtualBox, but it still failed. As before, part of the message was :
Run 'make oldconfig && make prepare' on kernel src to fix it.
only this time it worked (it did not before).
Another try of "/etc/rc.d/vboxdrv setup' now showed a completely different error message : all my source packages were for kernel 184.108.40.206 while I had a 220.127.116.11 kernel running.
So I used yast to install the 18.104.22.168 kernel, rebooted the PC, ran the '/etc/rc.d/vboxdrv setup' thing again and guess what : It worked !!!
Which means I have virtualbox installed now. Cannot wait to start playing around with it.