hw/i2c: Rename i2c_try_create_slave() as i2c_slave_new()

We use "new" names for functions that allocate and initialize
device objects: pci_new(), isa_new(), usb_new().
Let's call this one i2c_slave_new(). Since we have to update
all the callers, also let it return a I2CSlave object.

Suggested-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20200705224154.16917-3-f4bug@amsat.org>
Signed-off-by: Corey Minyard <cminyard@mvista.com>