tpm: Use /dev/null for cancel path if none was found

TPM 2 does not implement sysfs files for cancellation of commands.
We therefore use /dev/null for the cancel path passed to QEMU.
Signed-off-by: NStefan Berger <stefanb@linux.vnet.ibm.com>
Tested-by: NJavier Martinez Canillas <javierm@redhat.com>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: NJiri Denemark <jdenemar@redhat.com>
(cherry picked from commit dfbb15b7)

src/util/virtpm.c

@@ -61,9 +61,7 @@ virTPMCreateCancelPath(const char *devpath)
                 VIR_FREE(path);
             }
             if (!path)
-                virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
-                               _("No usable sysfs TPM cancel file could be "
-                                 "found"));
+                ignore_value(VIR_STRDUP(path, "/dev/null"));
         } else {
             virReportError(VIR_ERR_INTERNAL_ERROR,
                            _("TPM device path %s is invalid"), devpath);