tpm-backend: Initialize and free data members in it's own methods

Initialize and free TPMBackend data members in it's own instance_init() and
instance_finalize methods.

Took the opportunity to remove unneeded destroy() method from TpmDriverOps
interface as TPMBackend is a Qemu Object, we can use object_unref() inplace of
tpm_backend_destroy() to free the backend object, hence removed destroy() from
TPMDriverOps interface.

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>
