hw/i2c: add asynchronous send

Add an asynchronous version of i2c_send() that requires the slave to
explicitly acknowledge on the bus with i2c_ack().

The current master must use the new i2c_start_send_async() to indicate
that it wants to do an asynchronous transfer. This allows the i2c core
to check if the target slave supports this or not. This approach relies
on adding a new enum i2c_event member, which is why a bunch of other
devices needs changes in their event handling switches.

Signed-off-by: Klaus Jensen <k.jensen@samsung.com>
Message-Id: <20220601210831.67259-5-its@irrelevant.dk>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20220630045133.32251-6-me@pjd.dev>
Signed-off-by: Cédric Le Goater <clg@kaod.org>

hw/nvram/eeprom_at24c.c

@@ -75,6 +75,8 @@ int at24c_eeprom_event(I2CSlave *s, enum i2c_event event)
         break;
     case I2C_NACK:
         break;
+    default:
+        return -1;
     }
     return 0;
 }