conf: Introduce VIR_DOMAIN_TPM_MODEL_DEFAULT as default model

conf: Introduce VIR_DOMAIN_TPM_MODEL_DEFAULT as default model

Introduce VIR_DOMAIN_TPM_MODEL_DEFAULT as a default model which we use
in case the user does not provide a model in the device XML. It has
the TIS's previous value of '0'. In the post parsing function
we change this default value to 'TIS' to have the same model as before.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Ján Tomko <jtomko@redhat.com>