libqos: convert I2C to qgraph

Create an i2c-bus interface, corresponding to the I2CAdapter struct.
Wrap IMXI2C and OMAPI2C with a QOSGraphObject, and add the get_driver
function to retrieve the I2CAdapter.

The conversion is still not complete; for simplicity, i2c_recv and
i2c_send (along with their wrappers) still take an adapter/address
pair.  Fixing that would be complicated until the tests are converted
to qgraph, so it is left for after the conversion.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>