Don't verify CPU features with host-passthrough
    
    Commit fba6bc4 introduced the non-migratable invtsc feature,
    breaking save/migration with host-model and host-passthrough.
    
    On hosts with this feature present it was automatically included
    in the CPU definition, regardless of QEMU support.
    
    Commit de0aeaf stopped including it by default for host-model,
    but failed to fix host-passthrough.
    
    This commit ignores checking of CPU features with host-passthrough,
    since we don't pass them to QEMU (only -cpu host is passed),
    allowing domains using host-passthrough that were saved with
    the broken version of libvirtd to be restored.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1147584