[Libguestfs] Cached filesystem data

I am trying to use guestfs to mount, read-only, a VDI filesystem that is also open, read-write, by a separate Oracle VirtualBox VM. Opening the filesystem works fine, however changes made to the filesystem in the VirtualBox VM are not reflected in guestfs. Even unmounting and then mounting the drive again does not show the changes made since the first mount. However, shutdown() and then launch() of the guests does then show the changes. I have tried the drop_caches() call, but this doesn't help either (which is kind of unsurprising if unmounting is insufficient). Is there any way of causing guestfs to see the changes made to the filesystem without going through the slow shutdown/launch process? Cheers, Doug.