s390x/css: introduce and realize ccw-request callback

s390x/css: introduce and realize ccw-request callback

Introduce a new callback on subchannel to handle ccw-request.
Realize the callback in vfio-ccw device. Besides, resort to
the event notifier handler to handling the ccw-request results.
1. Pread the I/O results via MMIO region.
2. Update the scsw info to guest.
3. Inject an I/O interrupt to notify guest the I/O result.

Acked-by: Alex Williamson <[email protected]>
Signed-off-by: Xiao Feng Ren <[email protected]>
Signed-off-by: Dong Jia Shi <[email protected]>
Message-Id: <20170517004813[email protected]>
Signed-off-by: Cornelia Huck <[email protected]>