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

Signed-off-by: Hui Zhang <hui.zhang@citrix.com>
Looking at the other `FEATURE_`s in the context (shutdown, suspend, vcpu_hotplug) they all correspond to operations on the VM "virtual hardware". The XenAPI VM.* is mostly about manipulating and monitoring the "virtual hardware" and mostly not about performing operations within the guest operating system (although the implementation of `VM.clean_shutdown` happens to do this as part of its implementation it could have been implemented in terms of an ACPI button push). This means that the VM.* functions don't really care whether you're running Linux, FreeBSD or Windows.

Since this feature seems very Windows-dependent ("terminal services") and is tied to our particular guest agent (and is not part of the standard Xen guest interface which is more minimal), I think we should reflect that in the name. Perhaps we should call the API `VM_guest_agent.foo`?