vfio: Wrap vfio group module init/clean code into helpers

This wraps the init/clean code of vfio group global variable to be
helpers, and prepares for further moving vfio group specific code into
separate file.

As container is used by group, so vfio_container_init/cleanup() is moved
into vfio_group_init/cleanup().

Link: https://lore.kernel.org/r/20221201145535.589687-9-yi.l.liu@intel.com


Reviewed-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>