hw/i2c/core: Add i2c_try_create_slave() and i2c_realize_and_unref()

hw/i2c/core: Add i2c_try_create_slave() and i2c_realize_and_unref()

Extract i2c_try_create_slave() and i2c_realize_and_unref()
from i2c_create_slave().
We can now set properties on a I2CSlave before it is realized.

This is in line with the recent qdev/QOM changes merged
in commit 6675a653d2e.

Reviewed-by: Corey Minyard <cminyard@mvista.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Tested-by: Cédric Le Goater <clg@kaod.org>
Message-id: 20200623072723.6324-2-f4bug@amsat.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>