CP-10203: XenAPI: api call for RDP on/off request

Signed-off-by: Hui Zhang <hui.zhang@citrix.com>
Ah, I see, fair enough. However, this error name is part of the high-level XenAPI, and does not necessarily need to use the same terminology as the low-level xenstore interface. In fact, the API calls use "rdp" rather than "ts" already. From a XenAPI user's point of view, it may be clearer to use "rdp" in the error as well.