scsi_target_send_command(): amend stable-1.6 port of the CVE-2013-4344 fix

The originally suggested fix for CVE-2013-4344 introduced a regression in
scsi_target_send_command() / REQUEST_SENSE; the third argument passed to
scsi_device_get_sense() -- for the "len" parameter -- ignored the
possibility of the guest SCSI driver requesting truncated (or shorter than
full) sense data.

This could result in (r->len > req->cmd.xfer) on return, which is not
valid SCSI.

The problem was addressed in the second round, and the commit on the
master branch (84642435) is correct. However the stable-1.6 branch (the
v1.6.1 release) has the original, regressive fix (commit fdcbe7d5); let's
update it.

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

hw/scsi/scsi-bus.c

@@ -460,7 +460,8 @@ static int32_t scsi_target_send_command(SCSIRequest *req, uint8_t *buf)
         break;
     case REQUEST_SENSE:
         scsi_target_alloc_buf(&r->req, SCSI_SENSE_LEN);
-        r->len = scsi_device_get_sense(r->req.dev, r->buf, r->buf_len,
+        r->len = scsi_device_get_sense(r->req.dev, r->buf,
+                                       MIN(req->cmd.xfer, r->buf_len),
                                        (req->cmd.buf[1] & 1) == 0);
         if (r->req.dev->sense_is_ua) {
             scsi_device_unit_attention_reported(req->dev);