prep_pci: Convert to QOM realizefn

SysBusDeviceClass' initfn merely calls SysBusDeviceClass::init, so we
can already hook up our own realizefn overwriting this behavior.

A symmetric unrealizefn is not necessary, knowing that the child's
unrealizefn is still no-op, too. Avoids ripping it out again when
recursive realization at DeviceState-level is implemented.

Signed-off-by: Andreas Färber <andreas.faerber@web.de>
