Add functions to determine how port should be opened based on type.

Depending on the port and type of datapath, a port may need to be opened
as a different type of device than it's configured.  For example, an
"internal" port on a "dummy" datapath should opened as a "dummy" port.
This commit adds the ability for a dpif to provide this information to a
caller.  It will be used in a future commit.

Signed-off-by: Justin Pettit <jpettit@nicira.com>