qemu: driver: Allow cancellation of the backup job

Use the helper which cancels all blockjobs to perform the backup job
cancellation in qemuDomainAbortJob.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

src/qemu/qemu_driver.c

@@ -14044,7 +14044,8 @@ static int qemuDomainAbortJob(virDomainPtr dom)
         break;
 
     case QEMU_ASYNC_JOB_BACKUP:
-        /* TODO: to be implemented later */
+        qemuBackupJobCancelBlockjobs(vm, priv->backup, true);
+        ret = 0;
         break;
 
     case QEMU_ASYNC_JOB_LAST: