Update usage

Tell users they should specify the name for an image.

guestfs-inspectd.c

@@ -143,7 +143,7 @@ static void *worker_task(zsock_t *pipe, char *disk_path) {
 
 int main(int argc, char **argv) {
   if (argc < 2) {
-    printf("Usage: %s <disk-path> ...\n", argv[0]);
+    printf("Usage: %s <disk-path>:<name> ...\n", argv[0]);
     return EXIT_FAILURE;
   }