SITL: add support for multiple i2c buses

libraries/SITL/SIM_I2C.cpp

@@ -94,7 +94,7 @@ int I2C::ioctl_rdwr(i2c_rdwr_ioctl_data *data)
             }
         }
         if (!handled) {
-            ::fprintf(stderr, "Unhandled i2c message: addr=0x%x flags=%u len=%u\n", msg.addr, msg.flags, msg.len);
+            ::fprintf(stderr, "Unhandled i2c message: bus=%u addr=0x%02x flags=%u len=%u\n", msg.bus, msg.addr, msg.flags, msg.len);
             return -1;  // ?!
         }
     }

libraries/SITL/SIM_I2C.h

@@ -40,6 +40,7 @@ public:
 #define I2C_M_RD 1
 #define I2C_RDWR 0
     struct i2c_msg {
+        uint8_t bus;
         uint8_t addr;
         uint8_t flags;
         uint8_t *buf;