tests: add qmp_assert_error_class()

This helper will simplify a bunch of code checking for QMP errors and
can be shared by various tests.  Note that test-qga does check for
error description as well, so don't replace the code there for now.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
diff --git a/tests/test-qga.c b/tests/test-qga.c
index f69cdf6..3d63774 100644
--- a/tests/test-qga.c
+++ b/tests/test-qga.c
@@ -244,17 +244,12 @@
 static void test_qga_invalid_oob(gconstpointer fix)
 {
     const TestFixture *fixture = fix;
-    QDict *ret, *error;
-    const char *class;
+    QDict *ret;
 
     ret = qmp_fd(fixture->fd, "{'exec-oob': 'guest-ping'}");
     g_assert_nonnull(ret);
 
-    error = qdict_get_qdict(ret, "error");
-    class = qdict_get_try_str(error, "class");
-    g_assert_cmpstr(class, ==, "GenericError");
-
-    qobject_unref(ret);
+    qmp_assert_error_class(ret, "GenericError");
 }
 
 static void test_qga_invalid_args(gconstpointer fix)