hw/ppc/spapr_pci.c: Use device_cold_reset() rather than device_legacy_reset()

hw/ppc/spapr_pci.c: Use device_cold_reset() rather than device_legacy_reset()

In spapr_phb_children_reset() we call device_legacy_reset() to reset any
QOM children of the SPAPR PCI host bridge device. This will not reset
any qbus such a child might own. Switch to device_cold_reset(), which will
reset both the device and its buses. (If the child has no qbuses then
there will be no change in behaviour.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20221014142841.2092784-1-peter.maydell@linaro.org>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>