datapath-windows: Clarify externalVport.

In this patch, we add some explanation about the usage of
'externalVport' in the switch context. Also, we rename 'externalVport'
to 'virtualExternalVport' in alignment with the explanation. Also, we
rename 'numVports' to 'numHvVports' since ports are added from 2 ends
now.

Signed-off-by: Nithin Raju <nithin@vmware.com>
Acked-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Tested-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Acked-by: Ankur Sharma <ankursharma@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>