xen/balloon: Support xend-based toolstack

Xend-based toolstacks don't have static-max entry in xenstore. The
equivalent node for those toolstacks is memory_static_max.

Fixes: 5266b8e4445c (xen: fix booting ballooned down hvm guest)
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: <stable@vger.kernel.org> # 4.13
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/xen-balloon.c

@@ -76,12 +76,15 @@ static void watch_target(struct xenbus_watch *watch,
 
 	if (!watch_fired) {
 		watch_fired = true;
-		err = xenbus_scanf(XBT_NIL, "memory", "static-max", "%llu",
-				   &static_max);
-		if (err != 1)
-			static_max = new_target;
-		else
+
+		if ((xenbus_scanf(XBT_NIL, "memory", "static-max",
+				  "%llu", &static_max) == 1) ||
+		    (xenbus_scanf(XBT_NIL, "memory", "memory_static_max",
+				  "%llu", &static_max) == 1))
 			static_max >>= PAGE_SHIFT - 10;
+		else
+			static_max = new_target;
+
 		target_diff = (xen_pv_domain() || xen_initial_domain()) ? 0
 				: static_max - balloon_stats.target_pages;
 	}