Add a hasMacAddress member to InterfaceParams.

This is a convenience function for callers who would like to know
whether the interface has a MAC address. It is needed because the
constructed object will have a MAC address of 02:00:00:00:00:00
if the interface did not have a MAC address.

Test: added minimal unit test coverage
Change-Id: I422422d032afbbabcf8594def76702bb053f0a96