vfio: Rename __vfio_group_unset_container()

To vfio_group_detach_container(). This function is really a container
function.

Fold the WARN_ON() into it as a precondition assertion.

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/2-v3-297af71838d2+b9-vfio_container_split_jgg@nvidia.com


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