vfio-ccw: support async command subregion

vfio-ccw: support async command subregion

A vfio-ccw device may provide an async command subregion for
issuing halt/clear subchannel requests. If it is present, use
it for sending halt/clear request to the device; if not, fall
back to emulation (as done today).

Reviewed-by: Farhan Ali <alifm@linux.ibm.com>
Message-Id: <20190613092542.2834-1-cohuck@redhat.com>
Reviewed-by: Eric Farman <farman@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>