Thanks for all your answers.
I understand that what I want to achieve seemed pretty confused. I will try to clarify :
On real hardware, I have an I2C device used to get temperatures, pressure etc... and it works on x86 and there were no QEMU virtualized hardware corresponding.
I don't really need to simulate the I2C hardware in QEMU. Indeed, there are few of them just sending regular i2c data. For some tests, I want to be able to send/receive these ones from my daemon on the host. After some researches, what I was thinking about was :
1) Use virtio-serial and write an I2C driver (guest kernel) that would give/take data to/from i2c-1 and read/write to vportp0... Seemed a bit ugly, so I wanted to try something else.