qdev-monitor-test: Don't test human-readable error message

Test the error class instead.  Expecting a specific message is
fragile.  In fact, it broke once already, in commit 75884afd.  Restore
the test of error member "class" dropped there, and drop the test of
error member "desc".

There are no other tests of "desc" as far as I can tell.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NAndreas Färber <afaerber@suse.de>

tests/qdev-monitor-test.c

@@ -32,9 +32,7 @@ static void test_device_add(void)
                    "}}");
     g_assert(response);
     error = qdict_get_qdict(response, "error");
-    g_assert_cmpstr(qdict_get_try_str(error, "desc"),
-                    ==,
-                    "Device needs media, but drive is empty");
+    g_assert_cmpstr(qdict_get_try_str(error, "class"), ==, "GenericError");
     QDECREF(response);
 
     /* Delete the drive */