libvirt isn't just a wrapper around qemu-kvm, it drives half a dozen different virtualization systems, giving you the benefit of abstracting the details of the hypervisor from the management tools. So for that reason we don't support tweaking the qemu command line arbitrarily. However if there are particular features of qemu or KVM that you think we should support, please raise them.
I would like to see a feature equivalent to the -no-reboot in qemu. When I manually shutdown a virtual machine I don't like it automatically booting back up.