vmstat: Add root element to libxo output

Current libxo output does not have a root element. Valid XML requires a single
root element. This commit adds this root element.

The libxo output version bumped accordingly.

PR: 254635
Pull Request: https://github.com/freebsd/freebsd-src/pull/1330

(cherry picked from commit c7dd97ec99687996de49ec0b1a23bf007e5199b9)