Add ability to direct "packet-in"s to particular controllers.

Add ability to direct "packet-in"s to particular controllers.

Nicira's controllers are somewhat heterogeneous, so that particular
"packet-in" messages should be directed to particular controllers.  This
new Nicira extension action allows designating a controller or controllers
to receive the "packet-in" using a 16-bit integer ID.

The new NXAST_CONTROLLER action also specifies the "reason" code to include
in the "packet-in" message.  This is particularly useful for simulating a
"no-match" "packet-in" using a rule.

Feature #8946.
Signed-off-by: Ben Pfaff <blp@nicira.com>