This adds a new config option which is read on the destination host
during pre_live_migration and the value is returned back to the
source host, which can be used to determine, from the source host,
if it should wait for a "network-vif-plugged" event due to VIFs
being plugged on the destination host. This helps us to
avoid the guest transfer at all if vif plugging failed on the dest
host, which we just wouldn't find out until post live migration
and then we have to rollback.
The option is disabled by default for backward compatibility and
also because certain networking backends, like OpenDaylight, are
known to not send network-vif-plugged events unless the port host
binding information changes, which for live migration doesn't happen
until after the guest is transferred to the destination host.
We could arguably avoid the changes to the live migrate data
versioned object and just assume the same networking backend is
used within each cell, but this does allow the deployer to have
the flexibility of live migrating between different network
backends (eventually anyway). The ability to live migrate between
different VIF types is being worked on as part of blueprint
neutron-new-port-binding-api.
Related to blueprint neutron-new-port-binding-api
Change-Id: I0f3ab6604d8b79bdb75cf67571e359cfecc039d8
5aadff75c3