After qemu_iovec_destroy, the QEMUIOVector's size is zeroed and
the zero size ultimately is used to compute virtqueue_push's len
argument.  Therefore, reads from virtio-blk devices did not
migrate their results correctly.  (Writes were okay).

Save the size in virtio_blk_handle_request, and use it when the request
is completed.

Based on a patch by Wen Congyang.
Signed-off-by: NWen Congyang <wency@cn.fujitsu.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Tested-by: NLi Zhijian <lizhijian@cn.fujitsu.com>
Message-id: 1427997044-392-1-git-send-email-pbonzini@redhat.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>