virtio_pci: pm: Use CONFIG_PM_SLEEP instead of CONFIG_PM

virtio_pci: pm: Use CONFIG_PM_SLEEP instead of CONFIG_PM

The virtio_pci_freeze/restore are defined under CONFIG_PM but is used
by SET_SYSTEM_SLEEP_PM_OPS macro, which is defined under
CONFIG_PM_SLEEP. So if CONFIG_PM_SLEEP is not cofigured but
CONFIG_PM_RUNTIME is, the following warning message appeared:

drivers/virtio/virtio_pci.c:770:12: warning: ‘virtio_pci_freeze’ defined but not used [-Wunused-function]
 static int virtio_pci_freeze(struct device *dev)
            ^
drivers/virtio/virtio_pci.c:790:12: warning: ‘virtio_pci_restore’ defined but not used [-Wunused-function]
 static int virtio_pci_restore(struct device *dev)
            ^
Fix it by changing CONFIG_PM to CONFIG_PM_SLEEP.

Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Reviewed-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>