Commonly these elements won't show up to users.And in VMXML.__init__(),VMXML.xml's value is so simple that if I add these properties in VMXML, every accessor I wrote must be contain "set_cpu_mode() and new_from_dumpxml()", I think it's duplicate, so I add this new class that VMCPUXML.__init__ contains "set_cpu_mode() and dumpxml()", it's convenient for writing accessors.
I tried to modified VMXML.__init__(), but it's affect is so large, but add a new class inherit VMXML can be more easy and convenient.