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>