How do I make xm mem-set command permanent

Simple question. I have a couple of servers with 32GB of RAM. They are booting into xen. When I look in each virt-manager Domain-0 uses up all the RAM. (Actually around 31.2 GB). I issue the xm mem-set Domain-0 1024 command and it reduces to about 1.3GB
After rebooting the machines the Domain-0 again consumes all the RAM. How do I make the changes permanent? Is there a file I need to edit etc?

Just an idea: can you add that command to one of the boot scripts in /etc/init.d/ ?

When, then to /etc/iinit.d/boot.local and not to another one that may become overwritten at the first update.