During live-migration nova waits for 'network-vif-plugged' event
in order to proceeed with live-migration . While using OVN
it never happened because the original design assumes usage
of neutron agents.
This patch updates port status from DOWN to ACTIVE during
pre_live_migration state, which in fact for Neutron/ML2
is done by neutron-ovs-agent, just only to emit
'network-vif-plugged' notification and allow nova
to perform live-migration.
(cherry picked from commit 229f894ee6aff6b5da8cc399575537feed1c6a49)