qemu: Don't compare CPU against host for TCG
    
    Right now when building the qemu command line, we try to do various
    unconditional validations of the guest CPU against the host CPU. However
    this checks are overly applied. The only time we should use the checks
    are:
    
    - The user requests host-model/host-passthrough, or
    
    - When KVM is requsted. CPU features requested in TCG mode are always
      emulated by qemu and are independent of the host CPU, so no host CPU
      checks should be performed.
    
    Right now if trying to specify a CPU for arm on an x86 host, it attempts
    to do non-sensical validation and falls over.
    
    Switch all the test cases that were intending to test CPU validation to
    use KVM, so they continue to test the intended code.
    
    Amend some aarch64 XML tests with a CPU model, to ensure things work
    correctly.