balloon: Fix a misleading error message

The error message

    {"execute": "balloon", "arguments":{"value": -1}}
    {"error": {"class": "GenericError", "desc": "Parameter 'target' expects a size"}}

points to 'target' instead of 'value'.  Fix:

    {"error": {"class": "GenericError", "desc": "Parameter 'value' expects a size"}}

Root cause: qmp_balloon()'s parameter is named @target.  Rename it to
@value to match the QAPI schema.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20231031111059.3407803-7-armbru@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Mario Casquero <mcasquer@redhat.com>

system/balloon.c

@@ -90,17 +90,17 @@ BalloonInfo *qmp_query_balloon(Error **errp)
     return info;
 }
 
-void qmp_balloon(int64_t target, Error **errp)
+void qmp_balloon(int64_t value, Error **errp)
 {
     if (!have_balloon(errp)) {
         return;
     }
 
-    if (target <= 0) {
-        error_setg(errp, QERR_INVALID_PARAMETER_VALUE, "target", "a size");
+    if (value <= 0) {
+        error_setg(errp, QERR_INVALID_PARAMETER_VALUE, "value", "a size");
         return;
     }
 
-    trace_balloon_event(balloon_opaque, target);
-    balloon_event_fn(balloon_opaque, target);
+    trace_balloon_event(balloon_opaque, value);
+    balloon_event_fn(balloon_opaque, value);
 }