vfio/iommu_type1: Fix some sanity checks in detach group

vfio_sanity_check_pfn_list() is used to check whether pfn_list and
notifier are empty when remove the external domain, so it makes a
wrong assumption that only external domain will use the pinning
interface.

Now we apply the pfn_list check when a vfio_dma is removed and apply
the notifier check when all domains are removed.

Fixes: a54eb550 ("vfio iommu type1: Add support for mediated devices")
Signed-off-by: Keqian Zhu <zhukeqian1@huawei.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>