[libvirt] Why not support additional machine args in XML such as max-ram-below-4g?

Since qemu support the arg -machine like that: -machine [type=]name[,prop[=value][,...]] So I'm inquisitive about why libvirt doesn't support specify the prop in xml ? So I can specify the qemu machine args like -max-raw-below-4g in xml, well, I mean not by specifying the -qemu-command-line in xml. Thanks, Zongyong Wu