vfio: Record host IOMMU's available IO page sizes

Depending on the host IOMMU type we determine and record the available page
sizes for IOMMU translation.  We'll need this for other validation in
future patches.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
