hw/i2c: pmbus: reset page register for out of range reads

hw/i2c: pmbus: reset page register for out of range reads

The linux pmbus driver scans all possible pages and does not reset the
current page after the scan, making all future page reads fail as out of range
on devices with a single page.

This change resets out of range pages immediately on write.

Also added a qtest for simultaneous writes to all pages.

Reviewed-by: Hao Wu <wuhaotsh@google.com>
Signed-off-by: Titus Rwantare <titusr@google.com>
Message-ID: <20231023-staging-pmbus-v3-v4-8-07a8cb7cd20a@google.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>