intel_iommu: allow queued invalidation for IR

Queued invalidation is required for IR. This patch add basic support for
interrupt cache invalidate requests. Since we currently have no IR cache
implemented yet, we can just skip all interrupt cache invalidation
requests for now.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
