Revert "net: Do not fill legacy info_str for backends"

Several issues has been reported for query-netdev info
series. Consider it's late in the rc, this reverts commit
f2e8319d456724c3d8514d943dc4607e2f08e88a.

Signed-off-by: Jason Wang <jasowang@redhat.com>
diff --git a/net/slirp.c b/net/slirp.c
index a9fdc7a..67f0f1d 100644
--- a/net/slirp.c
+++ b/net/slirp.c
@@ -674,6 +674,9 @@
         stored->tftp_server_name = g_strdup(tftp_server_name);
     }
 
+    nc->info_str = g_strdup_printf("net=%s,restrict=%s", inet_ntoa(net),
+                                   restricted ? "on" : "off");
+
     s = DO_UPCAST(SlirpState, nc, nc);
 
     s->slirp = slirp_init(restricted, ipv4, net, mask, host,