vfio/type1: Use the new helper to find vfio_group

When attaching a new group to the container, let's use the new helper
vfio_iommu_find_iommu_group() to check if it's already attached. There
is no functional change.

Also take this chance to add a missing blank line.

Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>