CA-37279: Stunnel error from WLB connect "No route to host"

Understand "No route to host" as an expected error message from stunnel, and turn that into API error WLB_UNKNOWN_HOST if we see it when contacting WLB.

This is two patches, one for xen-api-libs.hg, and one for xen-api.hg.

Signed-off-by: Ewan Mellor <ewan.mellor@eu.citrix.com>

ocaml/xapi/workload_balancing.ml

@@ -305,6 +305,7 @@ let wlb_request ~__context ~host ~port ~auth ~meth ~params ~handler ~enable_log
           match error_msg with
           | "Connection refused" -> 
             raise_connection_refused ()
+          | "No route to host"
           | "No host resolved" -> 
             raise_unknown_host ()
           | "Invalid argument" ->