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>

chardev/char.c

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) {

include/chardev/char.h

diff --git a/include/chardev/char.h b/include/chardev/char.h
index 1604ea9..66dde46 100644
--- a/include/chardev/char.h
+++ b/include/chardev/char.h
@@ -65,7 +65,9 @@
  *
  * @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);

vl.c

diff --git a/vl.c b/vl.c
index dd803fc..99fcfa0 100644
--- a/vl.c
+++ b/vl.c
@@ -2344,10 +2344,12 @@
 {
     Error *local_err = NULL;
 
-    qemu_chr_new_from_opts(opts, &local_err);
-    if (local_err) {
-        error_report_err(local_err);
-        return -1;
+    if (!qemu_chr_new_from_opts(opts, &local_err)) {
+        if (local_err) {
+            error_report_err(local_err);
+            return -1;
+        }
+        exit(0);
     }
     return 0;
 }