nbd: Add writethrough to block-export-add

qemu-nbd allows use of writethrough cache modes, which mean that write
requests made through NBD will cause a flush before they complete.
Expose the same functionality in block-export-add.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200924152717.287415-10-kwolf@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

blockdev-nbd.c

@@ -216,9 +216,13 @@ BlockExport *nbd_export_create(BlockExportOptions *exp_args, Error **errp)
         goto out;
     }
 
+    if (!exp_args->has_writethrough) {
+        exp_args->writethrough = false;
+    }
+
     exp = nbd_export_new(bs, arg->name, arg->description, arg->bitmap,
                          !arg->writable, !arg->writable,
-                         NULL, false, errp);
+                         NULL, exp_args->writethrough, errp);
     if (!exp) {
         goto out;
     }