datapath: Merge "struct dp_port" into "struct vport".

datapath: Merge "struct dp_port" into "struct vport".

After the previous commit, which changed the datapath to always create and
attach a vport at the same time, and to always detach and delete a vport
at the same time, there is no longer any real distinction between a dp_port
and a vport.  This commit, therefore, merges the two together to simplify
code.  It might even improve performance, although I have not checked.

I wasn't sure at first whether the merged structure should be "struct
dp_port" or "struct vport".  I went with the latter since the "v" prefix
sounds cool.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>