virt: the description is an instance field, not a config key — set it with a PATCH

Found on the first live test on grid: incus config set refused 'description' and the whole
limits save with it.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

internal/api/virt.go

@@ -388,6 +388,12 @@ func (s *Server) virtConfig(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	var set, unset []string
+	desc, hasDesc := body["description"]
+	delete(body, "description")
+	if hasDesc && (len(desc) > 200 || strings.ContainsAny(desc, "\n\r")) {
+		jsonErr(w, 400, "bad value for description")
+		return
+	}
 	for k, v := range body {
 		v = strings.TrimSpace(v)
 		ok := false
@@ -400,8 +406,6 @@ func (s *Server) virtConfig(w http.ResponseWriter, r *http.Request) {
 			ok = incus.ValidCron(v)
 		case "snapshots.expiry":
 			ok = incus.ValidExpiry(v)
-		case "description":
-			ok = len(v) <= 200 && !strings.ContainsAny(v, "\n\r")
 		case "boot.autostart":
 			ok = v == "" || v == "true" || v == "false"
 		}
@@ -429,6 +433,14 @@ func (s *Server) virtConfig(w http.ResponseWriter, r *http.Request) {
 	for _, k := range unset {
 		_, _ = s.virt.Cmd(ctx, "config", "unset", name, k).CombinedOutput() // unsetting what isn't set is fine
 	}
+	if hasDesc {
+		// the description is a field of the instance, not a config key, so it goes in through the API
+		patch, _ := json.Marshal(map[string]string{"description": strings.TrimSpace(desc)})
+		if out, err := s.virt.Cmd(ctx, "query", "-X", "PATCH", "-d", string(patch), "/1.0/instances/"+name).CombinedOutput(); err != nil {
+			jsonErr(w, 409, strings.TrimPrefix(strings.TrimSpace(string(out)), "Error: "))
+			return
+		}
+	}
 	log.Printf("virt: config %s set %v unset %v (from %s)", name, set, unset, clientIP(r))
 	w.WriteHeader(204)
 }