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>
actually to be consistent, we should make the variable name separate by underscore:  port_mac.  Could you fix the other one too?