tpm-backend: Made few interface methods optional

This allows backend implementations left optional interface methods.
For mandatory methods assertion checks added.

Took the opportunity to remove unused methods:
 - tpm_backend_get_desc()
 - TPMDriverOps->handle_startup_error

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>
