configure: Fix configure output for RBD storage backend

We'd print status for the 'dir' backend instead of the correct one.

m4/virt-storage-rbd.m4

@@ -41,7 +41,7 @@ AC_DEFUN([LIBVIRT_STORAGE_CHECK_RBD], [
 ])
 
 AC_DEFUN([LIBVIRT_STORAGE_RESULT_RBD], [
-  LIBVIRT_RESULT([RBD], [$with_storage_dir])
+  LIBVIRT_RESULT([RBD], [$with_storage_rbd])
 ])
 
 AC_DEFUN([LIBVIRT_RESULT_RBD], [