libxl: create xenstore backend for empty disks

We still need to create the vbd backend in xenstore even if the disk
format is DISK_FORMAT_EMPTY.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/libxl/libxl.c

@@ -953,9 +953,7 @@ int libxl_device_disk_add(libxl_ctx *ctx, uint32_t domid, libxl_device_disk *dis
             break;
         case DISK_BACKEND_TAP:
         case DISK_BACKEND_QDISK: 
-            if (disk->format == DISK_FORMAT_EMPTY)
-                break;
-            if (libxl__blktap_enabled(&gc)) {
+            if (libxl__blktap_enabled(&gc) && disk->format != DISK_FORMAT_EMPTY) {
                 const char *dev = libxl__blktap_devpath(&gc,
                                                disk->pdev_path, disk->format);
                 if (!dev) {