util: Fix getting CPU frequency on Apple Silicon

util: Fix getting CPU frequency on Apple Silicon

The hw.cpufrequency sysctl, which we use to obtain the CPU
frequency on macOS, is not available when running on Apple
Silicon, and as a consequence we currently report an error
whenever such information is requested.

The virNodeInfo.mhz field, where the CPU frequency gets stored,
is documented as being zero when the information could not be
obtained, and we already do that for Linux on aarch64. Extend
this behavior to macOS on Apple Silicon.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>