tests: qemuxml2argv: fix expected type for usb-bus-missing

The guest of usb-bus-missing does not cause a parse error, but a
validation issue -- hence, switch from DO_TEST_PARSE_ERROR to
DO_TEST_FAILURE.

Fixes commit b003b978.
Signed-off-by: NPino Toscano <ptoscano@redhat.com>

tests/qemuxml2argvtest.c

@@ -1402,9 +1402,9 @@ mymain(void)
     DO_TEST("usb-port-missing",
             QEMU_CAPS_USB_HUB,
             QEMU_CAPS_NODEFCONFIG);
-    DO_TEST_PARSE_ERROR("usb-bus-missing",
-                        QEMU_CAPS_USB_HUB,
-                        QEMU_CAPS_NODEFCONFIG);
+    DO_TEST_FAILURE("usb-bus-missing",
+                    QEMU_CAPS_USB_HUB,
+                    QEMU_CAPS_NODEFCONFIG);
     DO_TEST("usb-ports",
             QEMU_CAPS_USB_HUB,
             QEMU_CAPS_NODEFCONFIG);