tpm-backend: store TPMIf interface, improve backend_init()

Store the TPM interface, the actual object may be different from
TPMState. Keep a reference on the interface, and check the backend
wasn't already initialized.

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>
