s390x/pci: make hot-unplug handler smoother

The current implementation of hot-unplug handler is abrupt. Any pci
operation will be just rejected if pci device is unconfigured. Thus a
pci device can not be reset or destroyed in a right, smooth and safe
way.

Improve this as follows:
- Notify the guest via a HP_EVENT_DECONFIGURE_REQUEST(0x303) event in
  the unplug handler, giving it a chance to deconfigure the device via
  sclp and allowing us to continue hot-unplug afterwards.
- Set up a timer that will generate the HP_EVENT_CONFIGURE_TO_STBRES
  (0x304) event as before if the guest did not react after an adequate
  time.

Signed-off-by: Yi Min Zhao <zyimin@linux.vnet.ibm.com>
Reviewed-by: Pierre Morel <pmorel@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>