Move thread handling inside TPMBackend, this way backend implementations need
not to maintain their own thread life cycle, instead they needs to implement
'handle_request()' class method that always been called from a thread.

This change made tpm_backend_int.h kind of useless, hence removed it.

Signed-off-by: Amarnath Valluri <amarnath.valluri@intel.com>
Reviewed-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>