iommu/vt-d, trivial: refine support of 64bit guest address

In Intel IOMMU driver, it calculate page table level from adjusted guest
address width as 'level = (agaw - 30) / 9', which assumes (agaw -30)
could be divided by 9. On the other hand, 64bit is a valid agaw and
(64 - 30) can't be divided by 9, so it needs special handling.

This patch enhances Intel IOMMU driver to correctly handle 64bit agaw.
It's mainly for code readability because there's no hardware supporting
64bit agaw yet.

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>