scsi-disk: Fix assertion failure on WRITE SAME

scsi-disk: Fix assertion failure on WRITE SAME

The last portion of an unaligned WRITE SAME command could fail the
assertion in bdrv_aligned_pwritev:

    assert(!qiov || bytes == qiov->size);

Because we updated data->iov.iov_len right above this if block, but
data->qiov still has the old size.

Reinitialize the qiov to make them equal and keep block layer happy.

Cc: [email protected]
Signed-off-by: Fam Zheng <[email protected]>
Message-Id: <1438159512[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>