IOMMU: page table dumping adjustments

IOMMU: page table dumping adjustments

For one none of the three IOMMU implementations on Arm specify a dumping
hook. Generalize VT-d's "don't dump shared page tables" to cover for
this.

Further in the past I was told that on Arm in principle there could be
multiple different IOMMUs, and hence different domains' platform_ops
pointers could differ. Use each domain's ops for calling the dump hook.
(In the long run all uses of iommu_get_ops() would likely need to
disappear for this reason.)

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>