VT-d: re-phrase logic in vtd_set_hwdom_mapping() for clarity

VT-d: re-phrase logic in vtd_set_hwdom_mapping() for clarity

It is hard to reconcile the comment at the top of the loop in
vtd_set_hwdom_mapping() with the if statement following it. This patch
re-phrases the logic, preserving the semantics, but making it easier
to read.

The patch also modifies the Xen command line documentation to make it
clear that iommu_inclusive_mapping only applies to pages up to the 4GB
boundary.

NOTE: This patch also corrects the indentation of the printk() towards
      the end of vtd_set_hwdom_mapping().

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Roger Pau Monne <roger.pau@citrix.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>