vfio/platform: Allow the selection of a given iommu backend

vfio/platform: Allow the selection of a given iommu backend

Now we support two types of iommu backends, let's add the capability
to select one of them. This depends on whether an iommufd object has
been linked with the vfio-platform device:

If the user wants to use the legacy backend, it shall not
link the vfio-platform device with any iommufd object:

 -device vfio-platform,host=XXX

This is called the legacy mode/backend.

If the user wants to use the iommufd backend (/dev/iommu) it
shall pass an iommufd object id in the vfio-platform device options:

 -object iommufd,id=iommufd0
 -device vfio-platform,host=XXX,iommufd=iommufd0

Suggested-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>