vmx: avoid memory leak

* src/vmx/vmx.c: fix memory leak, 'def' has a initial value 'NULL', so
  'goto cleanup' is perfected instead of adding a virConfFree before
  'return NULL'.
Signed-off-by: NAlex Jia <ajia@redhat.com>

src/vmx/vmx.c

@@ -1253,7 +1253,7 @@ virVMXParseConfig(virVMXContext *ctx, virCapsPtr caps, const char *vmx)
     /* Allocate domain def */
     if (VIR_ALLOC(def) < 0) {
         virReportOOMError();
-        return NULL;
+        goto cleanup;
     }
 
     def->virtType = VIR_DOMAIN_VIRT_VMWARE;