[Libvir] How to get the memory used by a guest domin

Hi, It appears to be no API available to provide the memory used by a guest domin. I looked at the availble libvirt api called "virNodeGetCellsFreeMemory" using which it is possible to get the free mem. But looking further I found that this is free memory of the "whole box" rather than based upon the guest domain. How can I get the memory used by a guest domin. Thanks n Regards, -Pranay.