Properly synchronize status for ports deleted from backend

Ensure that the status for a port is properly synchronized even
when the port itself does not exist anymore in the backend, avoiding
raising exceptions due to missing results in the NVP query.

This patch also adjusts a comment and improves exception handling
in _nvp_get_port_id

Bug 1229331

Change-Id: Ibaa581006c994a543c1795dd3f1d50086583b953
Merge "Properly synchronize status for ports deleted from backend"