hw/i2c: pmbus add support for block receive

hw/i2c: pmbus add support for block receive

PMBus devices can send and receive variable length data using the
block read and write format, with the first byte in the payload
denoting the length.

This is mostly used for strings and on-device logs. Devices can
respond to a block read with an empty string.

Reviewed-by: Hao Wu <wuhaotsh@google.com>
Acked-by: Corey Minyard <cminyard@mvista.com>
Signed-off-by: Titus Rwantare <titusr@google.com>
Message-ID: <20231023-staging-pmbus-v3-v4-1-07a8cb7cd20a@google.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>