vdpa: Use kernel coding style for structure comments

As subsequent patch adds new structure field with comment, move the
structure comment to follow kernel coding style.

Signed-off-by: Parav Pandit <parav@nvidia.com>
Reviewed-by: Eli Cohen <elic@nvidia.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20211026175519.87795-4-parav@nvidia.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>