vfio-ccw: Add a trace for asynchronous requests

Since the asynchronous requests are typically associated with
error recovery, let's add a simple trace when one of those is
issued to a device.

Signed-off-by: Eric Farman <farman@linux.ibm.com>
Message-Id: <20191016142040.14132-4-farman@linux.ibm.com>
Acked-by: Halil Pasic <pasic@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>