VGA console: one console operation per VM, a data websocket per SPICE channel

spice-html5 opens a websocket per channel (main, display, inputs, cursor, playback), and
each one was making Ham Lab open a new console operation, which Incus refuses as already
connected. Now the first channel opens the operation and holds its control websocket; every
further channel is one more data websocket on the same operation, which Incus allows; the
operation is let go when the last channel closes. A stale operation from a previous Ham Lab
process is taken over with force.

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

internal/api/virt.go

@@ -535,24 +535,14 @@ func (s *Server) virtVGA(w http.ResponseWriter, r *http.Request) {
 	conn.SetReadLimit(64 << 20)
 	ctx, cancel := context.WithCancel(r.Context())
 	defer cancel()
-	display, control, err := s.virt.VGA(ctx, name)
+	display, release, err := s.virt.VGAChannel(ctx, name)
 	if err != nil {
 		log.Printf("virt: vga %s refused: %v (from %s)", name, err, clientIP(r))
 		conn.Close(websocket.StatusInternalError, truncate(err.Error(), 120))
 		return
 	}
-	log.Printf("virt: vga console of %s opened (from %s)", name, clientIP(r))
+	defer release()
 	defer display.Close(websocket.StatusNormalClosure, "")
-	if control != nil {
-		defer control.Close(websocket.StatusNormalClosure, "")
-		go func() { // the control channel carries nothing we send; keep it open and drain it
-			for {
-				if _, _, err := control.Read(ctx); err != nil {
-					return
-				}
-			}
-		}()
-	}
 	go func() {
 		defer cancel()
 		for {
@@ -576,7 +566,6 @@ func (s *Server) virtVGA(w http.ResponseWriter, r *http.Request) {
 	}
 	cancel()
 	conn.Close(websocket.StatusNormalClosure, "")
-	log.Printf("virt: vga console of %s closed", name)
 }
 
 func truncate(s string, n int) string {