virtio-iommu: Introduce lazy mode domains and IOTLB_FLUSH request

[eauger]

At the moment the virtio-iommu offers MAP and UNMAP requests.
This means that if the virtio-iommu protects a device with an iotlb
we have no other choice but invalidating the device iotlb on UNMAP.
This proposal aims at allowing the decoupling of UNMAP commands
and domain IOTLB invalidations. This enables lazy mode where iotlb
invalidations are deferred and batched. In that prospect we add a
new VIRTIO_IOMMU_F_LAZY_IOTLB_FLUSH feature which, when offered
allows an endpoint to be attached to a domain in lazy mode. Alongside
we introduce a new IOTLB_FLUSH request which allows to invalidate
iotlb entries for the whole domain. When the domain is created in
lazy mode, the virtio-iommu device knows it will receive IOTLB_FLUSH
and it can avoid flushing the device IOTLB on individual UNMAPs.