Add utility function for checking trusted port

Add utility function for checking trusted port

Ports that have a device_owner that starts with 'network:'
are trusted in several places throughout the codebase. Each
of these did a startswith check on each field and it's not
immediately obvious why it's done.

This patch adds a utility function called 'is_port_trusted'
that performs the same check and makes it obvious what is
being done.

Change-Id: I542c753776d5cfb2fd736b25ea6e111867c89c89