vfio: factor out a vfio_group_find_or_alloc helper

Factor out a helper to find or allocate the vfio_group to reduce the
spagetthi code in vfio_register_group_dev a little.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/20210924155705.4258-5-hch@lst.de


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