vfio: centralize module refcount in subsystem layer

Remove code duplication and move module refcounting to the subsystem
module.

Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Link: https://lore.kernel.org/r/20210518192133.59195-2-mgurtovoy@nvidia.com


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