trivial: Cleanup a comment about a now removed libvirt version check
trivial: Cleanup a comment about a now removed libvirt version check

Some associated logic checking the underlying libvirt hypervisor being
used is also reworked to read more easily.

Change-Id: I9ad5c2c74ecfa7a2a07e2888b628d2ecd4534523

openstack/nova,mahak/nova,openstack/nova,mahak/nova,openstack/nova,mahak/nova