tpm-be: call request_completed() out of thread

Lift from the backend implementation the responsability to call the
request_completed() callback outside of thread context. This also
simplify frontend/interface work, as they no longer need to care
whether the callback is called from a different thread.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>