util: Make qemu_prealloc_mem() optionally consume a ThreadContext

... and implement it under POSIX. When a ThreadContext is provided,
create new threads via the context such that these new threads obtain a
properly configured CPU affinity.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Message-Id: <20221014134720.168738-6-david@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>

util/oslib-win32.c

@@ -269,7 +269,7 @@ int getpagesize(void)
 }
 
 void qemu_prealloc_mem(int fd, char *area, size_t sz, int max_threads,
-                       Error **errp)
+                       ThreadContext *tc, Error **errp)
 {
     int i;
     size_t pagesize = qemu_real_host_page_size();