tools/libxenstat: fix populating vbd.rd_sect

Fixes: 91c3e3dc ("tools/xentop: Display '-' when stats are not available.")
Signed-off-by: Richard Kojedzinszky <richard@kojedz.in>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
(cherry picked from commit 89d57f29)