add checksums to udp independent of /dev/vhost-net.

In newer kernel versions, the network devices added for lxc containers
do not get checksums added.  The result is the same as previously
occurred when vhost-net became prevalent in kvm guests.

Software that expects the checksums on packets will ignore them.  One example
of such software is udhcpc in cirros.  Without this change, cirros containers
in 3.13 kernels (Ubuntu 14.04) will fail to acquire an address via dhcp.

Closes-Bug: #1335193
Change-Id: Iba305fb7f8236955ca732e467db9e424535be93d
Merge "add checksums to udp independent of /dev/vhost-net."