hw/i2c: Introduce i2c_start_recv() and i2c_start_send()

hw/i2c: Introduce i2c_start_recv() and i2c_start_send()

To ease reviewing code using the I2C bus API, introduce the
i2c_start_recv() and i2c_start_send() helpers which don't
take the confusing 'is_recv' boolean argument.

Use these new helpers in the SMBus / AUX bus models.

Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Corey Minyard <cminyard@mvista.com>