libvirt: add capability to specify CPU model and topology with attr

you can now use the attribute cpu_mode to configure a guest CPU to be
as close to host CPU as possible
cpu_mode attr if present will be handed to the <cpu mode="attr_value"/>

you can inseat use cpu_match to specify the cpu features and topology

For more documentation please see libvirt docs:
http://libvirt.org/formatdomain.html#elementsCPU
userguide: add documetation to specify CPU model and topology with attr