x86/iommu: add common page-table allocator

x86/iommu: add common page-table allocator

Instead of having separate page table allocation functions in VT-d and AMD
IOMMU code, we could use a common allocation function in the general x86 code.

This patch adds a new allocation function, iommu_alloc_pgtable(), for this
purpose. The function adds the page table pages to a list. The pages in this
list are then freed by iommu_free_pgtables(), which is called by
domain_relinquish_resources() after PCI devices have been de-assigned.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>