ofproto-dpif: Make ofproto/trace a bit more like real packet translation.

Until now, ofproto/trace has looked up the flow itself.  xlate_actions()
can do the flow lookup internally and, since that is what happens when a
packet arrives, having it do its own packet lookup makes a lot of sense.

I noticed this in connection with the actset_output field, which
xlate_actions() should set to OFPP_UNSET at the beginning of translation
before looking up the flow.  ofproto/trace didn't do that, so it looked
up a rule with actset_output=0 instead.  By having xlate_actions() do the
lookup, the behavior can be consistent and correct.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>