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.

Closes-Bug: #1341465
Change-Id: I1d71c8a2cf8c2f71f0dbcfb54c9b347e24c03562
Merge "ofagent: Handle device name prefixes other than "tap""