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 b003b9781b6ae633cfe4fdf6b9620ca246fa2432.
Signed-off-by: Pino Toscano <ptoscano@redhat.com>