char: don't exit on hmp 'chardev-add help'

qemu_chr_new_from_opts() is used from both vl.c and hmp,
and it is quite confusing to see qemu suddenly exit after receiving a help
option in hmp.

Do exit(0) from vl.c instead.

Signed-off-by: Anton Nefedov <anton.nefedov@virtuozzo.com>
Message-Id: <1500977081-120929-1-git-send-email-anton.nefedov@virtuozzo.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/chardev/char.h

@@ -65,7 +65,9 @@ struct Chardev {
  *
  * @opts see qemu-config.c for a list of valid options
  *
- * Returns: a new character backend
+ * Returns: on success: a new character backend
+ *          otherwise:  NULL; @errp specifies the error
+ *                            or left untouched in case of help option
  */
 Chardev *qemu_chr_new_from_opts(QemuOpts *opts,
                                 Error **errp);