intel-iommu: Fix hardware_largepage_caps()

intel-iommu: Fix hardware_largepage_caps()

 - Don't use domain_get_iommu(). It'll BUG() when used on a VM domain.
 - Do actually check the iov_pfn to see whether it's suitably aligned.
 - "Simplify" the algorithm to give the compiler a chance to optimise.
   (Yes, this version is probably harder to understand. But that's what
    comments in the code are for.)
 - Inline it (it was previously not even static).

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>