backends/iommufd: Introduce TYPE_HOST_IOMMU_DEVICE_IOMMUFD[_VFIO] devices

backends/iommufd: Introduce TYPE_HOST_IOMMU_DEVICE_IOMMUFD[_VFIO] devices

TYPE_HOST_IOMMU_DEVICE_IOMMUFD represents a host IOMMU device under
iommufd backend. It is abstract, because it is going to be derived
into VFIO or VDPA type'd device.

It will have its own .get_cap() implementation.

TYPE_HOST_IOMMU_DEVICE_IOMMUFD_VFIO is a sub-class of
TYPE_HOST_IOMMU_DEVICE_IOMMUFD, represents a VFIO type'd host IOMMU
device under iommufd backend. It will be created during VFIO device
attaching and passed to vIOMMU.

It will have its own .realize() implementation.

Opportunistically, add missed header to include/sysemu/iommufd.h.

Suggested-by: Cédric Le Goater <clg@redhat.com>
Signed-off-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: Michael S. Tsirkin <mst@redhat.com>