intel-iommu: fail DEVIOTLB_UNMAP without dt mode

Merge Request Required Information

RHEL 7.9 kernel doesn't support DEVIOTLB and this case is broken with vhost and latest QEMU that supports it.

Bugzilla: https://bugzilla.redhat.com/2156876 Tested: by QE

Summary of Changes

To be able to use this kernel we need to disable dev-iotlb message with "device-iotlb=off" and based on that fail the dev-iotlb registration to allow vhost to fallback to the legacy solution (using IOMMU_NOTIFIER_UNMAP rather than IOMMU_NOTIFIER_DEVIOTLB_UNMAP for it's translation)