get_secret_value returned bytes, so the edit dialog showed b'cHc=' (the
bytes' repr) for the base64 value cHc=. Saving the dialog as shown fed
"b'cHc='" to b64decode, which dropped the quotes and the b and stored
other bytes: the secret silently changed (pw became m\xc1\xdc). The value
is now base64 text, and set_secret_value decodes with validate=True, so
a value that is not base64 is refused with a message instead. The field
had an invalid pattern ([a-zA-Z0-9]$+, ignored by browsers) and a 45
character limit; it takes base64 of any length.
create_secret wrote the usage value into the secret XML with an f-string,
unchecked: a quote, "<" or "&" broke the XML or added to it. The page's
pattern ([a-z0-9. ]+) was the only check and refused real values: a
volume secret's path ("/") and an iSCSI IQN ("-", ":"). The XML is now
built with lxml, so the value is text whatever it holds, and the input
keeps only required and the form's 100 characters; libvirt reports a
value it does not take.
A live test creates a volume, an iSCSI and a Ceph secret with real
values through the page.
