vfio: Split the register_device ops call into functions

This is a container item.

A following patch will move the vfio_container functions to their own .c
file.

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/7-v3-297af71838d2+b9-vfio_container_split_jgg@nvidia.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>