vhost-user-test: do not hang if chardev creation failed

Before the chardev name fix, the following error may happen: "attempt
to add duplicate property 'chr-test' to object (type 'container')",
due to races.

Sadly, error_vprintf() uses g_test_message(), so you have to use
read the cryptic --debug-log to see it. Later, it would make sense to
use g_critical() instead, and catch errors with
g_test_expect_message() (in glib 2.34).

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20180215212552.26997-5-marcandre.lureau@redhat.com>
Acked-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tests/vhost-user-test.c

@@ -494,6 +494,7 @@ static void test_server_create_chr(TestServer *server, const gchar *opt)
     chr = qemu_chr_new(server->chr_name, chr_path);
     g_free(chr_path);
 
+    g_assert_nonnull(chr);
     qemu_chr_fe_init(&server->chr, chr, &error_abort);
     qemu_chr_fe_set_handlers(&server->chr, chr_can_read, chr_read,
                              chr_event, NULL, server, NULL, true);