block migration: fix return value

Modify block_save_iterate() to return positive/zero/negative
(success/not done/failure) return status. The computation of
the blocks transferred (an int64_t) exceeds the size of an
int return value.

Signed-off-by: Gary R Hook <gary.hook@nimboxx.com>
Reviewed-by: ChenLiang <chenliang88@huawei.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1416958202-15913-1-git-send-email-gary.hook@nimboxx.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
