migration: Set downtime_start even for postcopy

Postcopy calculates its downtime separately.  It always sets
MigrationState.downtime properly, but not MigrationState.downtime_start.

Make postcopy do the same as other modes on properly recording the
timestamp when the VM is going to be stopped.  Drop the temporary variable
in postcopy_start() along the way.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <20231030163346.765724-2-peterx@redhat.com>