The reworked API is now called virCPUUpdate and it should change the
provided CPU definition into a one which can be consumed by the QEMU
command line builder:

    - host-passthrough remains unchanged
    - host-model is turned into custom CPU with a model and features
      copied from host
    - custom CPU with minimum match is converted similarly to host-model
    - optional features are updated according to host's CPU
Signed-off-by: NJiri Denemark <jdenemar@redhat.com>