cpu: Rework cpuUpdate

cpu: Rework cpuUpdate

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: Jiri Denemark <jdenemar@redhat.com>