storage plugins: en/decode volume notes as UTF-8

storage plugins: en/decode volume notes as UTF-8

When writing into the file, explicitly utf8 encode it, and then try
to utf8 decode it on read.

If the notes are not valid utf8, we assume they were iso-8859 encoded
and return as is.

Technically this is a breaking change, since there are iso-8859
comments that would successfully decode as utf8, for example: the
byte sequence "C2 A9" would be "£" in iso, but would decode to "£".

From what i can tell though, this is rather unlikely to happen for
"real world" notes, because the first byte would be in the range of
C0-F7 (which are mostly language dependent characters like "Â") and
the following bytes would have to be in the range of 80-BF, which are
only special characters like "£" (or undefined)

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>