vfio/container: Introduce an instance_init() handler This allows us to move the initialization code from vfio_container_init(), which we will soon remove. 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>