qga: Improve error for guest-set-user-password parameter @crypted

The Windows version of guest-set-user-password rejects argument
"crypted": true with the rather useless "this feature or command is
not currently supported".  Improve to "'crypted' must be off on this
host".

QERR_UNSUPPORTED is now unused.  Drop.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240911131206.2503035-3-armbru@redhat.com>
2 files changed