tools/oxenstored: Fix Oxenstored Live Update

tl;dr This hunk was part of the patch emailed to xen-devel, but was missing
from what ultimately got committed.

https://lore.kernel.org/xen-devel/4164cb728313c3b9fc38cf5e9ecb790ac93a9600.1610748224.git.edvin.torok@citrix.com/
is the patch in question, but was part of a series that had threading issues.
I have a vague recollection that I sourced the commits from a local branch,
which clearly wasn't as up-to-date as I had thought.

Either way, it's my fault/mistake, and this hunk should have been part of what
got comitted.

Fixes: 00c48f57ab ("tools/oxenstored: Start live update process")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>
(cherry picked from commit 7110192b1d)

tools/ocaml/xenstored/xenstored.ml

@@ -418,6 +418,11 @@ let _ =
 		rw_sock
 	) in
 
+	(* required for xenstore-control to detect availability of live-update *)
+	Store.mkdir store Perms.Connection.full_rights (Store.Path.of_string "/tool");
+	Store.write store Perms.Connection.full_rights
+		(Store.Path.of_string "/tool/xenstored") Sys.executable_name;
+
 	Sys.set_signal Sys.sighup (Sys.Signal_handle sighup_handler);
 	Sys.set_signal Sys.sigterm (Sys.Signal_handle (fun _ ->
 		info "Received SIGTERM";