This patch fixes the odd error reporting when trying to send a file
descriptor to the TPM emulator if one has not passed a valid chardev.

$ x86_64-softmmu/qemu-system-x86_64 -tpmdev emulator,id=tpm0
qemu-system-x86_64: -tpmdev emulator,id=tpm0: tpm-emulator: Failed to send CMD_SET_DATAFD: Success
qemu-system-x86_64: -tpmdev emulator,id=tpm0: tpm-emulator: Could not cleanly shutdown the TPM: Success

This is the new error report:

$ x86_64-softmmu/qemu-system-x86_64 -tpmdev emulator,id=tpm0
qemu-system-x86_64: -tpmdev emulator,id=tpm0: tpm-emulator: parameter 'chardev' is missing

This change does not hide the display of supported TPM types if a non-existent type is passed:

$ x86_64-softmmu/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):
 passthrough   Passthrough TPM backend driver
    emulator   TPM emulator backend driver

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>