vfio: More vfio_file_is_group() use cases

Replace further open coded tests with helper.

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/166516896843.1215571.5378890510536477434.stgit@omen
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>