Disk shows N/A in local server-stats widget after upgrading from v0.8.5 to v0.8.6

Description

After upgrading Glance from v0.8.5 to v0.8.6, the Disk metric in the server-stats widget shows N/A. It displayed disk usage correctly in v0.8.5.

Widget configuration:

  - type: server-stats
    servers:
      - type: local
        name: Services

Steps to reproduce:

1. Run Glance v0.8.5 with the configuration above and confirm disk usage is displayed.
2. Upgrade to v0.8.6 using the same widget configuration.
3. Check the Disk metric in the Services widget.