Add an I2C bus driver for virtio para-virtualization.

The controller can be emulated by the backend driver in
any device model software by following the virtio protocol.

The device specification can be found on
https://lists.oasis-open.org/archives/virtio-comment/202101/msg00008.html.

By following the specification, people may implement different
backend drivers to emulate different controllers according to
their needs.
Co-developed-by: NConghui Chen <conghui.chen@intel.com>
Signed-off-by: NConghui Chen <conghui.chen@intel.com>
Signed-off-by: NJie Deng <jie.deng@intel.com>
Reviewed-by: NViresh Kumar <viresh.kumar@linaro.org>
Tested-by: NViresh Kumar <viresh.kumar@linaro.org>
Acked-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NWolfram Sang <wsa@kernel.org>