drm/i915/gvt: Change from vfio_group_(un)pin_pages to vfio_(un)pin_pages

Use the existing vfio_device versions of vfio_(un)pin_pages(). There is no
reason to use a group interface here, kvmgt has easy access to a
vfio_device.

Delete kvmgt_vdev::vfio_group since these calls were the last users.

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Acked-by: Zhi Wang <zhi.a.wang@intel.com>
Link: https://lore.kernel.org/r/5-v4-8045e76bf00b+13d-vfio_mdev_no_group_jgg@nvidia.com


Signed-off-by: Alex Williamson <alex.williamson@redhat.com>