vfio/iommu_type1: Mantain a counter for non_pinned_groups

With this counter, we never need to traverse all groups to update
pinned_scope of vfio_iommu.

Suggested-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Keqian Zhu <zhukeqian1@huawei.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>