Fix pointer arithmetic instead of concat

`"my number " + 5` does not produces `"my number 5"` but rather `"mber "`.
If the number was greater than `len(string) + 2`
(pointing to the \0 and one past the end in authorized, more in UB)
the result would have been undefined behaviour.

Transforme the number to std::string using toString.

Signed-off-by: Kevin Rocard <kevin.rocard@intel.com>