Commit 90212b12 changed the OVS agent so adding vital drop flows on
br-int (table 0 priority 2) for packets from physical bridges was
deferred until DVR initialization later on. But if br-int has no flows
from a previous run (eg after host reboot), then these packets will hit
the NORMAL flow in table 60. And if there is more than one physical
bridge, then the physical interfaces from the different bridges are now
essentially connected at layer 2 and a network loop is possible in the
time before the flows are added by DVR. Also the DVR code won't add them
until after RPC calls to the server, so a loop is more likely if the
server is not available.
This patch restores adding these flows to when the physical bridges are
first configured. Also updated a comment that was no longer correct and
updated the unit test.
(cherry picked from commit c1a77ef8b7)