NSX-v plugin has support for provider network type 'portgroup'.
This patch adds support for portgroup type binding in DVS plugin.
Creating a portgroup type network refers to an existing dvportgroup
in vSphere. Deleting this network would not delete the dvportgroup
similar to NSX-v plugin.
This functionality is required to import VMs on vSphere
connected to an existing dvportgroup.
Change-Id: I6fd1f3efdd258b5d4d5042d0f76d0a4b52cd69ee