docs: kvm: vfio: Suggest KVM_DEV_VFIO_GROUP_ADD vs VFIO_GROUP_GET_DEVICE_FD ordering

as some vfio_device's open_device op requires kvm pointer and kvm pointer
set is part of GROUP_ADD.

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Yi Liu <yi.l.liu@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20230421053611.55839-1-yi.l.liu@intel.com


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