Fix typeerror issue for Filesystem.by_label("Root")

Change-Id: I40d46518a4883889919b58467f910c0351c44f3c
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1079695
Signed-off-by: hadong <hadong0720@gmail.com>
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>