char: Use error_printf() to print help and such

Calling error_report() in a function that takes an Error ** argument
is suspicious.  Convert a few that are actually help and such to
error_printf().

Improves output of -chardev help from

    qemu-system-x86_64: -chardev help: Available chardev backend types:
    serial
    ...

to

    Available chardev backend types:
    serial
    ...

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Marc-André Lureau" <marcandre.lureau@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20181017082702.5581-8-armbru@redhat.com>

chardev/char-pty.c

@@ -259,7 +259,7 @@ static void char_pty_open(Chardev *chr,
     qemu_set_nonblock(master_fd);
 
     chr->filename = g_strdup_printf("pty:%s", pty_name);
-    error_report("char device redirected to %s (label %s)",
+    error_printf("char device redirected to %s (label %s)\n",
                  pty_name, chr->label);
 
     s = PTY_CHARDEV(chr);

chardev/char.c

@@ -634,7 +634,7 @@ Chardev *qemu_chr_new_from_opts(QemuOpts *opts, Error **errp)
 
         chardev_name_foreach(help_string_append, str);
 
-        error_report("Available chardev backend types: %s", str->str);
+        error_printf("Available chardev backend types: %s\n", str->str);
         g_string_free(str, true);
         return NULL;
     }