vfio: Create host IOMMU device instance

vfio: Create host IOMMU device instance

Create host IOMMU device instance in vfio_attach_device() and call
.realize() to initialize it further.

Introuduce attribute VFIOIOMMUClass::hiod_typename and initialize
it based on VFIO backend type. It will facilitate HostIOMMUDevice
creation in vfio_attach_device().

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>