vbox: add version 6.0 CAPI header

Changes in the API:

 - The CreatedSharedFolder method now accepts a target mount
   point. Since we don't request automount, we're just passing
   NULL. We could, however, use this to pass the desired
   mount target from the XML config in future.
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>