versatile_pci: Update to realize and instance init functions

Update the Versatile PCI controller to use a realize function rather
than SysBusDevice::init. To reflect the fact that the 'realview_pci'
class is taking most of its implementation from 'versatile_pci' (and
to make the QOM casts work) we make 'realview_pci' a subclass of
'versatile_pci'.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Paul Brook <paul@codesourcery.com>