This makes the DMA unmap callback registration to container be consistent
across the vfio iommufd compat mode and the legacy container mode.

In the vfio iommufd compat mode, this registration is done in the
vfio_iommufd_bind() when creating access which has an unmap callback. This
is prior to calling the open_device() op. The existing mdev drivers have
been converted to be OK with this order. So it is ok to swap the order of
vfio_device_container_register() and open_device() for legacy mode.

This also prepares for further moving group specific code into separate
source file.

Link: https://lore.kernel.org/r/20221201145535.589687-6-yi.l.liu@intel.comReviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Tested-by: Lixiao Yang <lixiao.yang@intel.com>
Tested-by: Yu He <yu.he@intel.com>
Signed-off-by: Yi Liu <yi.l.liu@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>