Catch tracebacks when mounting invalid filesystem images under guestfs.

                         "with LIBGUESTFS_MEMSIZE=256 or lower.")

            return None

        devices = self.g.list_devices()

        try:

            self.g.mount(devices[0], '/')

        except RuntimeError:

            logger.exception("guestfs count not mount image; invalid file?")

            return None

        self.fs = self.g.list_filesystems()[devices[0]]

        return self