iscsi: ignore flushes on scsi-generic devices

Non-block SCSI devices do not support flushing, but we may still send
them requests via bdrv_flush_all.  Just ignore them.

Reviewed-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

block/iscsi.c

@@ -417,6 +417,10 @@ static int coroutine_fn iscsi_co_flush(BlockDriverState *bs)
     IscsiLun *iscsilun = bs->opaque;
     struct IscsiTask iTask;
 
+    if (bs->sg) {
+        return 0;
+    }
+
     iscsi_co_init_iscsitask(iscsilun, &iTask);
 
 retry: