CA-145413: xenopsd internal error: Unix.Unix_error(27, "rmdir", "/var/run/nonpersistent/xenops/extra")

FileFS.rm has a race condition in deleting emtpy dir when there're 2 or more vms are rebooting.
So let's use try-catch to delete empty dir.

Signed-off-by: Phus Lu <phus.lu@citrix.com>