virtio-scsi: Fix assert in virtio_scsi_push_event

Hotplugging a scsi-disk may trigger the assertion in qemu_sgl_concat.

    qemu-system-x86_64: qemu/hw/scsi/virtio-scsi.c:115: qemu_sgl_concat:
    Assertion `skip == 0' failed.

This is introduced by commit 55783a5 (virtio-scsi: work around bug in
old BIOSes) which didn't check out_num when accessing out_sg[0].iov_len
(the same to in sg). For virtio_scsi_push_event, looking into out_sg
doesn't make sense because 0 req_size is intended.

Cc: qemu-stable@nongnu.org
[Cc'ing qemu-stable because 55783a5 did it too]
Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <1426233354-525-1-git-send-email-famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/scsi/virtio-scsi.c

@@ -146,8 +146,12 @@ static int virtio_scsi_parse_req(VirtIOSCSIReq *req,
      * TODO: always disable this workaround for virtio 1.0 devices.
      */
     if (!virtio_has_feature(vdev, VIRTIO_F_ANY_LAYOUT)) {
-        req_size = req->elem.out_sg[0].iov_len;
-        resp_size = req->elem.in_sg[0].iov_len;
+        if (req->elem.out_num) {
+            req_size = req->elem.out_sg[0].iov_len;
+        }
+        if (req->elem.in_num) {
+            resp_size = req->elem.in_sg[0].iov_len;
+        }
     }
 
     out_size = qemu_sgl_concat(req, req->elem.out_sg,