Introducing macvtap device to netlink package

Small refactoring of MacVlan calls. Adds tests, too.

Signed-off-by: Milos Gajdos <milosgajdos83@gmail.com> (github: milosgajdos83)
Merge pull request #278 from milosgajdos83/macvtap

Introducing macvtap device to netlink package