iostatus: fix comments for block_job_iostatus_reset

Signed-off-by: Changlong Xie <xiecl.fnst@cn.fujitsu.com>
Message-id: 1464600491-23340-1-git-send-email-xiecl.fnst@cn.fujitsu.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>