Add QMP probing for TPM

Add QMP probing for TPM

Probe for QEMU's QMP TPM support by querying the lists of
supported TPM models (query-tpm-models) and backend types
(query-tpm-types).

The setting of the capability flags following the strings
returned from the commands above is only provided in the
patch where domain_conf.c gets TPM support due to dependencies
on functions only introduced there.

Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Reviewed-by: Corey Bryant <coreyb@linux.vnet.ibm.com>
Tested-by: Corey Bryant <coreyb@linux.vnet.ibm.com>