qemu: Enable 'host-passthrough' cpu mode for arm
    
    This patch allows libvirt user to specify 'host-passthrough'
    cpu mode while using qemu/kvm backend on arm (arm32).
    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: Oleg Strikov <oleg.strikov@canonical.com>