migration: Show downtime during postcopy phase

migration: Show downtime during postcopy phase

The downtime should be displayed during postcopy phase because the
switchover phase is done.  OTOH it's weird to show "expected downtime"
which can confuse what does that mean if the switchover has already
happened anyway.

This is a slight ABI change on QMP, but I assume it shouldn't affect
anyone.

Reviewed-by: Leonardo Bras <leobras@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>