qapi: Document DEVICE_TRAY_MOVED addition

Commit 2d76e724 failed to add a versioning tag to 'id'.

I audited all qapi*.json files from v2.7.0 to the current
state of the tree, and didn't find any other additions where
we failed to use a version tag.
Signed-off-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20161206160345.22425-1-eblake@redhat.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

qapi/block.json

@@ -199,7 +199,7 @@
 #          reasons, but it can be empty ("") if the image does not
 #          have a device name associated.
 #
-# @id: The name or QOM path of the guest device
+# @id: The name or QOM path of the guest device (since 2.8)
 #
 # @tray-open: true if the tray has been opened or false if it has been closed
 #