datapath-windows: Append flow attribute key

datapath-windows: Append flow attribute key

Currently when running the vswitch daemon we get a lot of messages of the
form:
2015-09-10T23:04:21Z|07255|dpif(revalidator11)|WARN|system@ovs-system: failed
to flow_del (Invalid argument).

The userspace expects after sending a delete flow command, to receive the flow
key of the deleted flow.

Currently we only send back the statiscs. This patch appends back the flow key
attribute for to the response buffer for the flow commands new, modify and
delete.

This patch also responds to the userspace with ENOENT in the case the flow
was not modified, deleted, created or retrieved.

Also incorporate some refactors.

Signed-off-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Acked-by: Sorin Vinturis <svinturis@cloudbasesolutions.com>
Acked-by: Sairam Venugopal <vsairam@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>