serial: initial qom-ification

serial: initial qom-ification

Make SerialState a device (the following patches will introduce IO/MM
sysbus serial devices)

None of the serial_{,mm}_init() callers actually free the returned
value (even if they did, it would be quite harmless), so we can change
the object allocation at will.

However, the devices that embed SerialState must now have their field
QOM-initialized manually (isa, pci, pci-multi).

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>