iommu/vt-d: Fix issue in computing domain's iommu_snooping flag

IOMMU units may dynamically attached to/detached from domains,
so we should scan all active IOMMU units when computing iommu_snooping
flag for a domain instead of only scanning IOMMU units associated
with the domain.

Also check snooping and superpage capabilities when hot-adding DMAR units.

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>