dpif-netdev: Per-port configurable EMC.

dpif-netdev: Per-port configurable EMC.

Conditional EMC insert helps a lot in scenarios with high numbers
of parallel flows, but in current implementation this option affects
all the threads and ports at once. There are scenarios where we have
different number of flows on different ports. For example, if one
of the VMs encapsulates traffic using additional headers, it will
receive large number of flows but only few flows will come out of
this VM. In this scenario it's much faster to use EMC instead of
classifier for traffic from the VM, but it's better to disable EMC
for the traffic which flows to VM.

To handle above issue introduced 'emc-enable' configurable to
enable/disable EMC on a per-port basis. Ex.:

  ovs-vsctl set interface dpdk0 other_config:emc-enable=false

EMC probability kept as is and it works for all the ports with
'emc-enable=true'.

Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Acked-by: Kevin Traynor <ktraynor@redhat.com>
Signed-off-by: Ian Stokes <ian.stokes@intel.com>