vfio: Selective dirty page tracking if IOMMU backed device pins pages

Added a check such that only singleton IOMMU groups can pin pages.
>From the point when vendor driver pins any pages, consider IOMMU group
dirty page scope to be limited to pinned pages.

To optimize to avoid walking list often, added flag
pinned_page_dirty_scope to indicate if all of the vfio_groups for each
vfio_domain in the domain_list dirty page scope is limited to pinned
pages. This flag is updated on first pinned pages request for that IOMMU
group and on attaching/detaching group.

Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
Reviewed-by: Neo Jia <cjia@nvidia.com>
Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>