Use variadic argument lists in various Helpers.

Various Helper functions take Attribute name/Value pairs with fixed numbers of arguments. Refactor to use variadic templates for arbitrary numbers of arguments.

QosWaveMacHelper::SetType() and NqosWaveMacHelper::SetType() were virtual, but then they can't be templated to handle variadic argument lists. The base class is not virtual, and nothing derives from these classes, so virtual doesn't seem to be required, and so was removed.