Personally I'd move the register_device() and unregister_device() functions outside of the VFS class entirely, perhaps to namespace scope.

Alternatively, there are other ways to leverage friendship and access control in C++. Here's one option:

    template <typename Registry>
    class DeviceManager;

    class Device {
        template<typename Registry> friend class DeviceManager;
        int y;
    };

    class VFS {
        friend struct DeviceManager<VFS>;
        int x;
    };

    template <typename Registry>
    struct DeviceManager {
        static void register_device (Registry& registry, Device& device) {
            registry.x = 1; // accessing privates
            device.y = 2;   // accessing privates
        }
    };

    int main() {
        VFS vfs;
        Device dev;
        DeviceManager<VFS>::register_device (vfs, dev);
    }

Here all DeviceManager<>'s can reach inside Devices, but only the VFS device manager can reach inside VFS.

This is also flexible. Remove 'static' and you get yourself a stateful mediator/manager. Add a variadic template register_devices_s_ member to Devicemanager and you've got yourself a convenience function for registering multiple devices (of heterogeneous types) without bloating the VFS API and, without runtime cost, and without losing type safety (by e.g. casting down to Device& from USBDevice&).