x86/iommu: don't map RAM holes above 4G

x86/iommu: don't map RAM holes above 4G

Matching the comment in iommu_set_dom0_mapping(), map only actual RAM
from the address range starting at 4G. It's not clear though whether
that comment is actually correct (which is why I'm sending this as
RFC), but it is certain that on systems with sparse physical memory
map we're currently wasting a potentially significant amount of memory
for setting up IOMMU page tables that will never be used.

The main question is what happens for MMIO ranges living above 4G. Of
course, the same issue would currently exist for any such ranges
sitting beyon the end of RAM.

Signed-off-by: Jan Beulich <jbeulich@novell.com>