VFIO: Wrapper for getting reference to vfio_device

- Added vfio_device_get_from_dev() as wrapper to get
  reference to vfio_device from struct device.

- Added vfio_device_data() as a wrapper to get device_data from
  vfio_device.

Signed-off-by: Vijay Mohan Pandarathil <vijaymohan.pandarathil@hp.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>