After some update the Virtualbox guests wouldn't start anymore. After some
trial and error I found in the Virtualbox log:

00:00:00.011066 main     rtldrNativeLoad:
dlopen('/usr/lib/virtualbox/libpthread.so', RTLD_NOW | RTLD_LOCAL) failed:
/usr/lib/virtualbox/libpthread.so: cannot open shared object file: No such file
or directory
00:00:00.011130 main     rtldrNativeLoad:
dlopen('/usr/lib/virtualbox/libdl.so', RTLD_NOW | RTLD_LOCAL) failed:
/usr/lib/virtualbox/libdl.so: cannot open shared object file: No such file or
directory

So I created a symbolic link in /usr/lib/virtualbox/ pointing to the current
version of those missing libs giving the link the expected name, after which I
was able to start a guest OS.

This is obviously a hack and there is probably a better way, so maybe someone
can fix it