nbd/server: Advertise all contexts in response to bare LIST

The NBD spec, and even our code comment, says that if the client
asks for NBD_OPT_LIST_META_CONTEXT with 0 queries, then we should
reply with (a possibly-compressed representation of) ALL contexts
that we are willing to let them try.  But commit 3d068af forgot
to advertise qemu:dirty-bitmap:FOO.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20181130023232.3079982-2-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

nbd/server.c

@@ -978,6 +978,7 @@ static int nbd_negotiate_meta_queries(NBDClient *client,
     if (client->opt == NBD_OPT_LIST_META_CONTEXT && !nb_queries) {
         /* enable all known contexts */
         meta->base_allocation = true;
+        meta->bitmap = !!meta->exp->export_bitmap;
     } else {
         for (i = 0; i < nb_queries; ++i) {
             ret = nbd_negotiate_meta_query(client, meta, errp);