backends: Introduce HostIOMMUDevice abstract

backends: Introduce HostIOMMUDevice abstract

A HostIOMMUDevice is an abstraction for an assigned device that is protected
by a physical IOMMU (aka host IOMMU). The userspace interaction with this
physical IOMMU can be done either through the VFIO IOMMU type 1 legacy
backend or the new iommufd backend. The assigned device can be a VFIO device
or a VDPA device. The HostIOMMUDevice is needed to interact with the host
IOMMU that protects the assigned device. It is especially useful when the
device is also protected by a virtual IOMMU as this latter use the translation
services of the physical IOMMU and is constrained by it. In that context the
HostIOMMUDevice can be passed to the virtual IOMMU to collect physical IOMMU
capabilities such as the supported address width. In the future, the virtual
IOMMU will use the HostIOMMUDevice to program the guest page tables in the
first translation stage of the physical IOMMU.

Introduce .realize() to initialize HostIOMMUDevice further after instance init.

Suggested-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>