virtio-blk: fix comment for virtio_blk_rw_complete as nalloc is initially -1

The initial value of nalloc is -1, but not 1.

Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-id: 1541479952-32355-1-git-send-email-dongli.zhang@oracle.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>