chardev/mux: convert size members to unsigned int

chardev/mux: convert size members to unsigned int

There is no sense to keep `focus`, `mux_cnt`, `prod`, `cons`
and `tag` variables as signed, those represent either size,
either position in array, which both are unsigned.

`focus` member of `MuxChardev` is kept signed, because initially
set to -1.

Signed-off-by: Roman Penyaev <r.peniaev@gmail.com>
Cc: "Marc-André Lureau" <marcandre.lureau@redhat.com>
Cc: qemu-devel@nongnu.org
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-ID: <20241014152408.427700-5-r.peniaev@gmail.com>