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.