utils.text: Function to verify MAC address prefix

The network management code needs to verify a MAC address prefix.
Instead of (ab)using NormalizeAndValidateMac, clean code should be used.
Unit tests for NormalizeAndValidateMac are updated and new ones for
NormalizeAndValidateThreeOctetMacPrefix are added.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>