Add port_mac method to Port

This patch adds the missing port_mac setter method to Port class.

This fixes MN-1911.

Signed-off-by: Takaaki Suzuki <captain@midokura.com>
Signed-off-by: Taku Fukushima <tfukushima@midokura.com>
Merge pull request #177 from tfukushima/mn-1911-v1.4

Add port_mac method to Port