vfio/iommu_type1: rename vfio_group struck to vfio_iommu_group

The vfio_group structure is already defined in vfio module so in order
to improve code readability and for simplicity, rename the vfio_group
structure in vfio_iommu_type1 module to vfio_iommu_group.

Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Link: https://lore.kernel.org/r/20210608112841.51897-1-mgurtovoy@nvidia.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>