virfirewall: Fir a memleak in virFirewallParseXML()

As a part of parsing XML, virFirewallParseXML() calls
virXMLNodeContentString() and then passes the return value
further. But virXMLNodeContentString() is documented so that it's
the caller's responsibility to free the returned string, which
virFirewallParseXML() never does. This leads to a memory leak:

  14,300 bytes in 220 blocks are definitely lost in loss record 1,879 of 1,891
     at 0x4841858: malloc (vg_replace_malloc.c:442)
     by 0x5491E3C: xmlBufCreateSize (in /usr/lib64/libxml2.so.2.12.6)
     by 0x54C2401: xmlNodeGetContent (in /usr/lib64/libxml2.so.2.12.6)
     by 0x49F7791: virXMLNodeContentString (virxml.c:354)
     by 0x4979F25: virFirewallParseXML (virfirewall.c:1134)
     by 0x4ABEB1E: virNetworkLoadState (virnetworkobj.c:938)
     by 0x4ABF35C: virNetworkObjLoadAllState (virnetworkobj.c:1072)
     by 0x4E9A25E: networkStateInitialize (bridge_driver.c:624)
     by 0x4CB1FA6: virStateInitialize (libvirt.c:665)
     by 0x15A6C6: daemonRunStateInit (remote_daemon.c:611)
     by 0x49E69F0: virThreadHelper (virthread.c:256)
     by 0x532B428: start_thread (in /lib64/libc.so.6)

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

src/util/virfirewall.c

@@ -1137,7 +1137,7 @@ virFirewallParseXML(virFirewall **firewall,
                                            NULL, NULL, NULL);
             for (i = 0; i < nargs; i++) {
 
-                char *arg = virXMLNodeContentString(argsNodes[i]);
+                g_autofree char *arg = virXMLNodeContentString(argsNodes[i]);
                 if (!arg)
                     return -1;