network: fix testsuite regression

I slightly botched commit be9fb5af - I converted '--arg=value' to
'--arg value', which has no semantic change, but did trip up the
testsuite.

* src/network/bridge_driver.c (networkBuildDnsmasqArgv): Restore
expected output.

src/network/bridge_driver.c

@@ -520,8 +520,7 @@ networkBuildDnsmasqArgv(virNetworkObjPtr network,
         int i;
 
         for (i = 0; i < dns->ntxtrecords; i++) {
-            virCommandAddArg(cmd, "--txt-record");
-            virCommandAddArgFormat(cmd, "%s,%s",
+            virCommandAddArgFormat(cmd, "--txt-record=%s,%s",
                                    dns->txtrecords[i].name,
                                    dns->txtrecords[i].value);
         }