qemu: command: explicitly error for non-x86 default CPU
    
    The code only currently handles writing an x86 default -cpu
    argument, and doesn't know anything about other architectures.
    Let's make this explicit rather than leaving ex. qemu ppc64 to
    throw an error about -cpu qemu64
    
    Reviewed-by: Ján Tomko <jtomko@redhat.com>
    Signed-off-by: Cole Robinson <crobinso@redhat.com>