vbox6: clear screen on guest triggered blanking

An invalid bitmap is exactly once set during blanking, use this as hint
to clear the GUI content. Same as done in vbox5.

Fixes #5263

repos/ports/src/virtualbox6/include/fb.h

@@ -214,6 +214,7 @@ class Genodefb :
 			Lock();
 
 			if (_display_bitmap.isNull()) {
+				_clear_screen();
 				Unlock();
 				return S_OK;
 			}