conf: cpu: Fix memory leak when specifying cpu vendor_id manually

The field was not freed from the cpu definition.

src/conf/cpu_conf.c

@@ -87,6 +87,7 @@ virCPUDefFree(virCPUDefPtr def)
         VIR_FREE(def->cells[i].cpustr);
     }
     VIR_FREE(def->cells);
+    VIR_FREE(def->vendor_id);
 
     VIR_FREE(def);
 }