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>

vl.c

@@ -4051,7 +4051,7 @@ int main(int argc, char **argv, char **envp)
     }
 
     if (cpu_model && is_help_option(cpu_model)) {
-        list_cpus(stdout, &fprintf, cpu_model);
+        list_cpus(cpu_model);
         exit(0);
     }