hw/s390x/s390-pci-inst.c: Use device_cold_reset() to reset PCI devices

hw/s390x/s390-pci-inst.c: Use device_cold_reset() to reset PCI devices

The semantic difference between the deprecated device_legacy_reset()
function and the newer device_cold_reset() function is that the new
function resets both the device itself and any qbuses it owns,
whereas the legacy function resets just the device itself and nothing
else.

In s390-pci-inst.c we use device_legacy_reset() to reset an
S390PCIBusDevice.  This device doesn't have any child qbuses, so the
functions do the same thing and we can stop using the deprecated one.

Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>