Modify is_valid_mac() for support unicode strings

Because an address can be used as <id> in ports REST API
we need also validation for mac in unicode strings.

Missing tests for is_valid_mac() added.

Change-Id: I7207c0fb9c1c2c1720337c1a7568f6b9cb8ea3c3
Merge "Modify is_valid_mac() for support unicode strings"