kvm/vfio: Store the struct file in the kvm_vfio_group

kvm/vfio: Store the struct file in the kvm_vfio_group

Following patches will change the APIs to use the struct file as the handle
instead of the vfio_group, so hang on to a reference to it with the same
duration of as the vfio_group.

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Yi Liu <yi.l.liu@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/2-v3-f7729924a7ea+25e33-vfio_kvm_no_group_jgg@nvidia.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>