common/pick_address.cc: silence GCC warning

common/pick_address.cc: silence GCC warning

* use `string_view` instead of string
* use `auto` instead of `int` to silence the warning
* use `enum class` instead of plain `enum`
* do not specify the value of enum, as
  we don't care about it

this change silences following warning

```
../src/common/pick_address.cc: In function ‘int get_iface_numa_node(const string&, int*)’:
../src/common/pick_address.cc:517:11: warning: comparison of integer expressions of different signedness: ‘int’ and ‘const size_type’ {aka ‘const long unsigned int’} [-Wsign-com\
pare]
  517 |   if (pos != string::npos ) {
      |       ~~~~^~~~~~~~~~~~~~~
```

Signed-off-by: Kefu Chai <kchai@redhat.com>