iommu: Make sysfs robust for non-API groups

Groups created by VFIO backends outside the core IOMMU API should never
be passed directly into the API itself, however they still expose their
standard sysfs attributes, so we can still stumble across them that way.
Take care to consider those cases before jumping into our normal
assumptions of a fully-initialised core API group.

Fixes: 3f6634d9

 ("iommu: Use right way to retrieve iommu_ops")
Reported-by: Jan Stancek <jstancek@redhat.com>
Tested-by: Jan Stancek <jstancek@redhat.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/86ada41986988511a8424e84746dfe9ba7f87573.1651667683.git.robin.murphy@arm.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>