This will prevent ovs agent from endless fail loop when dealing
with unbound port: like when port was created in neutron before
agent become alive, then agent gets online and and starts processing
devices.
This patch adds exception handling to prepare_port_filter() -
same as done in update_port_filter().
Change-Id: I1137eb18efaf51c67fab145e645f58cbd3772e40
Closes-Bug: #1849098
(cherry picked from commit e801159003)
(cherry picked from commit 0fb7f8ddac)
879bb90328