libxl: fix regression introduced in 5c883cf036

A few lines were erroneously deleted during rebase which caused domain
destruction to fail.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Tested-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/libxl/libxl_device.c

@@ -734,6 +734,11 @@ int libxl__device_destroy(libxl__gc *gc, libxl__device *dev)
     uint32_t domid;
     int libxl_only = dev->backend_kind == LIBXL__DEVICE_KIND_NONE;
 
+    if (!libxl_only) {
+        be_path = libxl__device_backend_path(gc, dev);
+        fe_path = libxl__device_frontend_path(gc, dev);
+    }
+
     rc = libxl__get_domid(gc, &domid);
     if (rc) goto out;