virtualbox: Fix load of dbus library at runtime.

VirtualBox had support for DBUS even in version 4.x, but it appears that
nothing in our VM test triggered it to load, thus I didn't notice the
runtime error:

rtldrNativeLoad: dlopen('libdbus-1.so.3', RTLD_NOW | RTLD_LOCAL) failed:
                 libdbus-1.so.3: cannot open shared object file: No such
                 file or directory

The upstream commits I think are responsible for this to come to surface
are _probably_ (did I ever mention that I love SVN? *cough*) one of
these:

https://www.virtualbox.org/changeset/55664/vbox
https://www.virtualbox.org/changeset/55602/vbox



Signed-off-by: aszlig <aszlig@redmoonstudios.org>