mpath: Fix memory leak in virStorageBackendCreateVols

@map_device, allocated by virAsprintf in virStorageBackendCreateVols,
was not freed and leaked.
Signed-off-by: NZhipeng Lu <lu.zhipeng@zte.com.cn>

src/storage/storage_backend_mpath.c

@@ -200,6 +200,7 @@ virStorageBackendCreateVols(virStoragePoolObjPtr pool,
 
     retval = 0;
  out:
+    VIR_FREE(map_device);
     return retval;
 }