enable hairpin mode on virtual interface bridge port

This is to support being able to DNAT/MASQ traffic from a container back into itself (dotcloud/docker#4442)

Docker-DCO-1.1-Signed-off-by: Patrick Hemmer <patrick.hemmer@gmail.com> (github: phemmer)
Merge pull request #62 from phemmer/docker/4442-hairpin-nat

enable hairpin mode on virtual interface bridge port