vfio: clean up the check for mediated device in vfio_iommu_type1

vfio: clean up the check for mediated device in vfio_iommu_type1

Pass the group flags to ->attach_group and remove the messy check for
the bus type.

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/20210924155705.4258-12-hch@lst.de
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>