libxl: always set a default value for VFB.opengl

Fixes an assertion failure.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Reported-by: Fantu <fantonifabio@tiscali.it>
Tested-by: Fantu <fantonifabio@tiscali.it>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/libxl/libxl.c

@@ -2256,9 +2256,7 @@ int libxl__device_vfb_setdefault(libxl__gc *gc, libxl_device_vfb *vfb)
     }
 
     libxl_defbool_setdefault(&vfb->sdl.enable, false);
-    if (libxl_defbool_val(vfb->sdl.enable)) {
-        libxl_defbool_setdefault(&vfb->sdl.opengl, false);
-    }
+    libxl_defbool_setdefault(&vfb->sdl.opengl, false);
 
     return 0;
 }