virtio-blk: report non-zero status when failing SG_IO requests

virtio-blk: report non-zero status when failing SG_IO requests

Linux really looks only at scsi->errors for SG_IO requests; it does
not look at the virtio request status at all.  Because of this, when
a SG_IO request is failed early with virtio_blk_req_complete(req,
VIRTIO_BLK_S_UNSUPP), without writing hdr.status, it will look like
a success to the guest.

This is their bug, but we can make it safe for older guests now by
forcing scsi->errors to have a non-zero value whenever a request
has to be failed.

But if we fix the bug in the guest driver, we will have another problem
because QEMU returns VIRTIO_BLK_S_IOERR if the status is non-zero, and
Linux translates that to -EIO.  Rather, the guest should succeed the
request and pass the non-zero status via the userspace-provided SG_IO
structure.  So, remove the case where virtio_blk_handle_scsi can
return VIRTIO_BLK_S_IOERR.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>