xenconfig: Resolve Coverity RESOURCE_LEAK

Since '337a13628' - Coverity complains that 'net' is VIR_ALLOC()'d, but
on various 'cleanup' exit paths from the code there is no corresponding
cleanup.

src/xenconfig/xen_common.c

@@ -960,6 +960,7 @@ xenParseVif(virConfPtr conf, virDomainDefPtr def)
     return 0;
 
  cleanup:
+    virDomainNetDefFree(net);
     VIR_FREE(script);
     return -1;
 }