util: Cleanup and rename os_mem_prealloc()

Let's
* give the function a "qemu_*" style name
* make sure the parameters in the implementation match the prototype
* rename smp_cpus to max_threads, which makes the semantics of that
  parameter clearer

... and add a function documentation.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Message-Id: <20221014134720.168738-2-david@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>