ofproto-dpif-xlate: Set up 'base_flow' when we initialize 'ctx'.

ofproto-dpif-xlate: Set up 'base_flow' when we initialize 'ctx'.

The initialization of 'base_flow' was previously split into a few pieces,
and I think it's easier to understand if it's all in one place.

This also moves and rewrites the comment describing 'base_flow'.  I think
that the perspective of the new comment is a little more useful.

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