Fix client API to support generic type

Fix client API to support generic type

- example -
VirtualTable<Policy<int>> table;
for(const auto& row : table) {
  Policy<int> policy = { row[&Policy<int>::name], row[&Policy<int>::value] };
  INFO(VIST_CLIENT) << "Policy table:";
  INFO(VIST_CLIENT) << "\t name: " << policy.name;
  INFO(VIST_CLIENT) << "\t value: " << policy.value;
}

Signed-off-by: Sangwan Kwon <sangwan.kwon@samsung.com>