Enable qemu libvirt_type on kvm compute nodes

For kvm compute nodes, libvirt_type must be set to 'qemu'. The
default is 'kvm'. quickstack::params::use_qemu_for_poc must be set to
true to activate this patch.
Merge pull request #120 from bcrochet/enable-qemu-virt

Enable qemu libvirt_type on kvm compute nodes