limit the amount of space admin is willing to assign to a base image copied locally

since the /var/lib/nova/instances/_base/ consumes the same storage as the instances we can see cases that because of old images there is not enough space for instances to run.
Although we have a way to configure a cleaning of old images I think that it would be nice to be able to create a separate share/lun or limit the %/size an admin will allow for base images.
I think it would be a nice addition to nova if we can create a separate share/luns for the base images so that do not consume storage form the instances.