char: Make -chardev help print to stdout

Command line help explicitly requested by the user should be printed
to stdout, not stderr.  We do elsewhere.  Adjust -chardev to match:
use qemu_printf() instead of error_printf().  Plain printf() would be
wrong because we need to print to the current monitor for "chardev-add
help".

Cc: "Marc-André Lureau" <marcandre.lureau@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190417190641.26814-14-armbru@redhat.com>