libxl: initialize sdl.opengl defbool in libxlMakeVfb

Commit bf32462b missed initializing sdl.opengl.  Without the
initialization, libvirtd will be terminated by an assert from libxl:

Assertion `!libxl_defbool_is_default(db)' failed.
Reported-by: NOlaf Hering <olaf@aepfle.de>

src/libxl/libxl_conf.c

@@ -1243,6 +1243,7 @@ libxlMakeVfb(virPortAllocatorPtr graphicsports,
         case VIR_DOMAIN_GRAPHICS_TYPE_SDL:
             libxl_defbool_set(&x_vfb->sdl.enable, 1);
             libxl_defbool_set(&x_vfb->vnc.enable, 0);
+            libxl_defbool_set(&x_vfb->sdl.opengl, 0);
             if (VIR_STRDUP(x_vfb->sdl.display, l_vfb->data.sdl.display) < 0)
                 return -1;
             if (VIR_STRDUP(x_vfb->sdl.xauthority, l_vfb->data.sdl.xauth) < 0)