libxl: initialize vfb defbools in libxlMakeVfb

If the domU configu has sdl enabled libvirtd crashes:
libvirtd[5158]: libvirtd: libxl.c:343: libxl_defbool_val:
Assertion `!libxl_defbool_is_default(db)' failed.

Initialize the relevant defbool variables in libxl_device_vfb.
Signed-off-by: NOlaf Hering <olaf@aepfle.de>
Cc: Jim Fehlig <jfehlig@suse.com>

src/libxl/libxl_conf.c

@@ -1242,6 +1242,7 @@ libxlMakeVfb(virPortAllocatorPtr graphicsports,
     switch (l_vfb->type) {
         case VIR_DOMAIN_GRAPHICS_TYPE_SDL:
             libxl_defbool_set(&x_vfb->sdl.enable, 1);
+            libxl_defbool_set(&x_vfb->vnc.enable, 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)
@@ -1249,6 +1250,7 @@ libxlMakeVfb(virPortAllocatorPtr graphicsports,
             break;
         case  VIR_DOMAIN_GRAPHICS_TYPE_VNC:
             libxl_defbool_set(&x_vfb->vnc.enable, 1);
+            libxl_defbool_set(&x_vfb->sdl.enable, 0);
             /* driver handles selection of free port */
             libxl_defbool_set(&x_vfb->vnc.findunused, 0);
             if (l_vfb->data.vnc.autoport) {