migration: Route errors down through migration_channel_connect

Route async errors (especially from sockets) down through
migration_channel_connect and on to migrate_fd_connect where they
can be cleaned up.

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