dmi data: fix failure of reading dmi data for unset dmi values

it is not uncommon to find dmi data in /sys full of 'ff'. utf-8
decoding of those would fail, causing warning and stacktrace.

Return '.' instead of \xff. This maps to what dmidecode would return

$ dmidecode --string system-product-name
.................................

1 files changed, 10 insertions, 3 deletions

diff --git a/cloudinit/util.py b/cloudinit/util.py
index e7407ea4..1d50edc9 100644
--- a/cloudinit/util.py
+++ b/cloudinit/util.py
@@ -2140,13 +2140,20 @@ def _read_dmi_syspath(key):
             LOG.debug("did not find %s", dmi_key_path)
             return None
 
-        key_data = load_file(dmi_key_path)
+        key_data = load_file(dmi_key_path, decode=False)
         if not key_data:
             LOG.debug("%s did not return any data", dmi_key_path)
             return None
 
-        LOG.debug("dmi data %s returned %s", dmi_key_path, key_data)
-        return key_data.strip()
+        # in the event that this is all \xff and a carriage return
+        # then return '.' in its place.
+        if key_data == b'\xff' * (len(key_data) - 1) + b'\n':
+            key_data = b'.' * (len(key_data) - 1) + b'\n'
+
+        str_data = key_data.decode('utf8').strip()
+
+        LOG.debug("dmi data %s returned %s", dmi_key_path, str_data)
+        return str_data
 
     except Exception:
         logexc(LOG, "failed read of %s", dmi_key_path)