The qemu volume parameter cache='' should be configurable when creating KVM hosts though libvirt.

When unset it defaults to write-through cache which is not optimal for RAW storage volumes:
http://www.linux-kvm.org/page/Tuning_KVM

It is configured in the " <driver name='qemu' type='<%= vol.format_type >'/>" part in < volumes.each do |vol| -%> .
https://github.com/fog/fog-libvirt/blob/master/lib/fog/libvirt/models/compute/templates/server.xml.erb:

I think it should to be able to set this options for volumes in the web gui and through the API.