This avoids opening group specific code in __vfio_register_dev() for the
sanity check if an (existing) group is not corrupted by having two copies
of the same struct device in it. It also simplifies the error unwind for
this sanity check since the failure can be detected in the group
allocation.

This also prepares for moving the group specific code into separate
group.c.

Grabbed from:
https://lore.kernel.org/kvm/20220922152338.2a2238fe.alex.williamson@redhat.com/

Link: https://lore.kernel.org/r/20221201145535.589687-3-yi.l.liu@intel.comReviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Tested-by: Lixiao Yang <lixiao.yang@intel.com>
Tested-by: Yu He <yu.he@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Yi Liu <yi.l.liu@intel.com>