target: Simplify how the TARGET_cpu_list() print

The various TARGET_cpu_list() take an fprintf()-like callback and a
FILE * to pass to it.  Their callers (vl.c's main() via list_cpus(),
bsd-user/main.c's main(), linux-user/main.c's main()) all pass
fprintf() and stdout.  Thus, the flexibility provided by the (rather
tiresome) indirection isn't actually used.

Drop the callback, and call qemu_printf() instead.

Calling printf() would also work, but would make the code unsuitable
for monitor context without making it simpler.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190417191805.28198-10-armbru@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

linux-user/main.c

@@ -317,7 +317,7 @@ static void handle_arg_cpu(const char *arg)
     if (cpu_model == NULL || is_help_option(cpu_model)) {
         /* XXX: implement xxx_cpu_list for targets that still miss it */
 #if defined(cpu_list)
-        cpu_list(stdout, &fprintf);
+        cpu_list();
 #endif
         exit(EXIT_FAILURE);
     }