Fix get_vif_port_by_id to only return relevant ports

This is returning any port, even if it's not on the switch that we're
looking at. As a side-effect, this means that we can actually manipulate
these ports while we really shouldn't.

Backported from 9bc2920

Change-Id: Ia4f4e93237c1c2ea6cb4b6c2f5adf78d6b34c7bf
Closes-Bug: #1283765
Merge "Fix get_vif_port_by_id to only return relevant ports" into stable/havana
Merge branch 'stable/havana' of https://github.com/openstack/quantum into havana/stable