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

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>