* @VIRTIO_PCIDEV_OP_CFG_READ: read config space, size is 1, 2, 4 or 8;
* the @data field should be filled in by the device (in little endian).
* @VIRTIO_PCIDEV_OP_CFG_WRITE: write config space, size is 1, 2, 4 or 8;
* the @data field contains the data to write (in little endian).
* @VIRTIO_PCIDEV_OP_CFG_READ: read config space, size is 1, 2, 4 or 8;
* the @data field should be filled in by the device (in little endian).
* @VIRTIO_PCIDEV_OP_CFG_WRITE: write config space, size is 1, 2, 4 or 8;
* the @data field contains the data to write (in little endian).