fix: assorted bug fixes for CH migration

Fix resource leaks, race conditions, and error handling across host
agent and control plane: proper sparse file cleanup on close error,
connect error wrapping for MakeDir, CoW file cleanup on pause failure,
per-sandbox VM directories, deferred map deletion to avoid race in VM
destroy, and goroutine launch for extension background workers.

internal/devicemapper/devicemapper.go

@@ -379,5 +379,9 @@ func createSparseFile(path string, sizeBytes int64) error {
 		os.Remove(path)
 		return err
 	}
-	return f.Close()
+	if err := f.Close(); err != nil {
+		os.Remove(path)
+		return err
+	}
+	return nil
 }