ocaml/xs: prefer using character device

Since 9c89dc9520 libxenstore prefers using
/dev/xen/xenbus over /proc/xen/xenbus. This makes the OCaml xenstore
library contain the same preference.

Signed-off-by: Doug Goldstein <cardoe@cardoe.com>
Acked-by: David Scott <dave.scott@citrix.com>

tools/ocaml/libs/xs/xs.ml

@@ -162,7 +162,13 @@ let daemon_open () =
 	with _ -> raise Failed_to_connect
 
 let domain_open () =
-	let path = "/proc/xen/xenbus" in
+	let path = try
+		let devpath = "/dev/xen/xenbus" in
+		Unix.access devpath [ Unix.F_OK ];
+		devpath
+	with Unix.Unix_error(_, _, _) ->
+		"/proc/xen/xenbus" in
+
 	let fd = Unix.openfile path [ Unix.O_RDWR ] 0o550 in
 	Unix.set_close_on_exec fd;
 	make fd