migration: Make sure that we don't call write() in case of error

If we are exiting due to an error/finish/.... Just don't try to even
touch the channel with one IO operation.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>