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>
diff --git a/chardev/char.c b/chardev/char.c
index c34b44a..5d283b9 100644
--- a/chardev/char.c
+++ b/chardev/char.c
@@ -620,7 +620,7 @@
 
         error_report("Available chardev backend types: %s", str->str);
         g_string_free(str, true);
-        exit(0);
+        return NULL;
     }
 
     if (id == NULL) {