vfio: Accept vfio device file in the KVM facing kAPI

vfio: Accept vfio device file in the KVM facing kAPI

This makes the vfio file kAPIs to accept vfio device files, also a
preparation for vfio device cdev support.

For the kvm set with vfio device file, kvm pointer is stored in struct
vfio_device_file, and use kvm_ref_lock to protect kvm set and kvm
pointer usage within VFIO. This kvm pointer will be set to vfio_device
after device file is bound to iommufd in the cdev path.

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Tested-by: Terrence Xu <terrence.xu@intel.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Tested-by: Matthew Rosato <mjrosato@linux.ibm.com>
Tested-by: Yanting Jiang <yanting.jiang@intel.com>
Tested-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.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/20230718135551.6592-4-yi.l.liu@intel.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>