tpm-passthrough: don't save guessed cancel_path in options

The value is later unneeded, and may leak if the free visitor doesn't
consider it since has_cancel_path is false. And for consistency with
"path" it shouldn't be returned in get_tpm_options().

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