virHostCPUGetInfo: Fix build on non-Unix like systems
    
    This function is plenty of ifdefs providing implementations for
    Linux, *BSD and OS-X. However, if we are being build for any
    other architecture, all that's left behind by preprocessor is
    just a error reporting call and return of -1. In that case,
    passed arguments are unused:
    
    ../../src/util/virhostcpu.c: In function 'virHostCPUGetInfo':
    ../../src/util/virhostcpu.c:966:33: error: unused parameter 'cpus' [-Werror=unused-parameter]
                       unsigned int *cpus,
                                     ^~~~
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>