dpif: Change provider interface to consistently use operation structs.

dpif: Change provider interface to consistently use operation structs.

Until now, a "flow put" has represented its parameters in two different
ways, depending on whether it was coming from dpif_flow_put() or from
dpif_operate(), and similarly for an "execute" operation.  This commit
adopts the operation struct consistently within the dpif provider
interface, which seems cleaner.

This commit also factors out logging for flow puts and executes, which
is useful in the following commit.

This doesn't change the dpif client interface, since the two forms are
more convenient for clients than always filling out an operation struct.

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