usb: mtp: tag root property as experimental

Reason: we don't want commit to that interface yet.  Possibly
the implementation will be switched over to use fsdev.
Suggested-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/usb/dev-mtp.c

@@ -1090,7 +1090,7 @@ static const VMStateDescription vmstate_usb_mtp = {
 };
 
 static Property mtp_properties[] = {
-    DEFINE_PROP_STRING("root", MTPState, root),
+    DEFINE_PROP_STRING("x-root", MTPState, root),
     DEFINE_PROP_STRING("desc", MTPState, desc),
     DEFINE_PROP_END_OF_LIST(),
 };