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

Signed-off-by: Hui Zhang <hui.zhang@citrix.com>
Here we read and write Xenstore nodes directly from xapi.
Wherever possible we want to do this only through xenopsd, so this function should just call a corresponding new function in xenopsd.
(Sorry I missed this on Friday when I was reviewing your earlier version of the code.)