intel_iommu_accel: Stick to system MR for IOMMUFD backed host device when x-flts=on

intel_iommu_accel: Stick to system MR for IOMMUFD backed host device when x-flts=on

When guest enables scalable mode and setup first stage page table, we don't
want to use IOMMU MR but rather continue using the system MR for IOMMUFD
backed host device.

Then default HWPT in VFIO contains GPA->HPA mappings which could be reused
as nesting parent HWPT to construct nested HWPT in vIOMMU.

Move vtd_as_key into intel_iommu_internal.h as it's also used by accel code.

Suggested-by: Yi Liu <yi.l.liu@intel.com>
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Yi Liu <yi.l.liu@intel.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20260106061304.314546-12-zhenzhong.duan@intel.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>