About Libvirt Setmem&dommemstat Function

[?9?7]

I am using 'virsh setmem' to adjust the vm memory. But I don't know what is the 
lowerlimit memory that can be adjusted to, so I try to update the version of 
libvirt&qemu to get caches of the vm memory so that I can calculate the 
lowerlimit. And when I upgrade libvirt to 4.10.0, qemu upgrade to 4.1.0, still, 
I cannot get 'stat-disk-caches'.  How can I solve it ? Thx.