ofagent: Handle device name prefixes other than "tap"

This fixes regressions in commit 9d13ea884bff749b3975acb5eb5630e5aca4a665.

Handle device name prefixes other than "tap".
For example, nova hybrid interface driver uses "qvo" prefix.

Also, ignore non neutron ports correctly.  For example, veth pairs
used to connect physical bridges.

Change-Id: I1d71c8a2cf8c2f71f0dbcfb54c9b347e24c03562