migration: register_savevm_live doesn't need dev

Commit 78dd48df removed the last caller of register_savevm_live for an
instantiable device (rather than a single system wide device);
so trim out the parameter.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20190822115433.12070-1-dgilbert@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>