vfio/container: Move vfio_container_destroy() to an instance_finalize() handler

vfio/container: Move vfio_container_destroy() to an instance_finalize() handler

vfio_container_destroy() clears the resources allocated
VFIOContainerBase object. Now that VFIOContainerBase is a QOM object,
add an instance_finalize() handler to do the cleanup. It will be
called through object_unref().

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