job: add .cancel handler for the driver

To be used in mirror in the following commit to cancel in-flight io on
target to not waste the time.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210205163720.887197-5-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>

include/qemu/job.h

@@ -251,6 +251,11 @@ struct JobDriver {
      */
     void (*clean)(Job *job);
 
+    /**
+     * If the callback is not NULL, it will be invoked in job_cancel_async
+     */
+    void (*cancel)(Job *job);
+
 
     /** Called when the job is freed */
     void (*free)(Job *job);

job.c

@@ -715,6 +715,9 @@ static int job_finalize_single(Job *job)
 
 static void job_cancel_async(Job *job, bool force)
 {
+    if (job->driver->cancel) {
+        job->driver->cancel(job);
+    }
     if (job->user_paused) {
         /* Do not call job_enter here, the caller will handle it.  */
         if (job->driver->user_resume) {