Help is a bit awkward when no TPM types are built into QEMU:

    $ qemu-system-x86_64 -tpmdev nonexistent,id=tpm0
    qemu-system-x86_64: -tpmdev nonexistent,id=tpm0: Parameter 'type' expects a TPM backend type
    Supported TPM types (choose only one):

Improve to

    qemu-system-x86_64: -tpmdev nonexistent,id=tpm0: Parameter 'type' expects a TPM backend type
    No TPM backend types are available

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>