vfio: Mark cdev usage in vfio_device

This can be used to differentiate whether to report group_id or devid in
the revised VFIO_DEVICE_GET_PCI_HOT_RESET_INFO ioctl. At this moment, no
cdev path yet, so the vfio_device_cdev_opened() helper always returns false.
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Tested-by: Yanting Jiang <yanting.jiang@intel.com>
Tested-by: Terrence Xu <terrence.xu@intel.com>
Tested-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Signed-off-by: Yi Liu <yi.l.liu@intel.com>
Link: https://lore.kernel.org/r/20230718105542.4138-7-yi.l.liu@intel.comSigned-off-by: Alex Williamson <alex.williamson@redhat.com>

include/linux/vfio.h

@@ -139,6 +139,11 @@ int vfio_iommufd_emulated_attach_ioas(struct vfio_device *vdev, u32 *pt_id);
 	((int (*)(struct vfio_device *vdev, u32 *pt_id)) NULL)
 #endif
 
+static inline bool vfio_device_cdev_opened(struct vfio_device *device)
+{
+	return false;
+}
+
 /**
  * struct vfio_migration_ops - VFIO bus device driver migration callbacks
  *