selftests: openvswitch: add flow dump support

Add a basic set of fields to print in a 'dpflow' format.  This will be
used by future commits to check for flow fields after parsing, as
well as verifying the flow fields pushed into the kernel from
userspace.

Signed-off-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>