ui/vt100: Take byte as uint8_t in bh_utf8_decode()

The bh_utf8_decode() UTF8 decoder takes its next byte as a "uint32_t
byte" parameter, but it assumes it to be in bounds as it immediately
indexes into its array with it.

Use "uint8_t" as the argument type instead. This moves us away from
the upstream implementation slightly, but it is the same type as
we use in the one callsite, and it makes it clear that we can't
be indexing off the end of the array with this guest-derived data.

This probably helps make Coverity a bit happier (CID 1659590).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-ID: <20260512104210.3330835-3-peter.maydell@linaro.org>