network: Fix printing XML 'delay' attribute

When specifying bridge delay via network XML define, we were looking for
the 'delay' attribute, but would dump the value as 'forwardDelay'. Have
the output match the expected input (and schema).

src/conf/network_conf.c

@@ -632,7 +632,7 @@ char *virNetworkDefFormat(virConnectPtr conn,
     virBufferAddLit(&buf, "  <bridge");
     if (def->bridge)
         virBufferEscapeString(&buf, " name='%s'", def->bridge);
-    virBufferVSprintf(&buf, " stp='%s' forwardDelay='%ld' />\n",
+    virBufferVSprintf(&buf, " stp='%s' delay='%ld' />\n",
                       def->stp ? "on" : "off",
                       def->delay);