scsi: Make device scsi-disk reject /dev/sg*

You're supposed to use scsi-generic for that.  Which rejects anything
but /dev/sg*.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

hw/scsi-disk.c

@@ -1031,6 +1031,11 @@ static int scsi_disk_initfn(SCSIDevice *dev)
     }
     s->bs = s->qdev.conf.dinfo->bdrv;
 
+    if (bdrv_is_sg(s->bs)) {
+        qemu_error("scsi-disk: unwanted /dev/sg*\n");
+        return -1;
+    }
+
     if (bdrv_get_type_hint(s->bs) == BDRV_TYPE_CDROM) {
         s->cluster_size = 4;
     } else {