This patch allows libvirt user to specify 'host-passthrough'
cpu mode while using qemu/kvm backend on aarch64.
It uses 'host' as a CPU model name instead of some other stub
(correct CPU detection is not implemented yet) to allow libvirt
user to specify 'host-model' cpu mode as well.
Signed-off-by: NOleg Strikov <oleg.strikov@canonical.com>

(crobinso: fix some indentation)