So that it doesn't have to be zeroed at each call.

Suggested-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Message-Id: <1474432046-325-3-git-send-email-famz@redhat.com>