hw/i2c: Rename i2c_create_slave() as i2c_slave_create_simple()

We use "create_simple" names for functions that allocate, initialize,
configure and realize device objects: pci_create_simple(),
isa_create_simple(), usb_create_simple(). For consistency, rename
i2c_create_slave() as i2c_slave_create_simple(). Since we have
to update all the callers, also let it return a I2CSlave object.

Suggested-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Corey Minyard <cminyard@mvista.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20200705224154.16917-5-f4bug@amsat.org>
Signed-off-by: Corey Minyard <cminyard@mvista.com>