qemuBlockJobRefreshJobs: Warn readers that 'job' may be invalid after update

Add a comment noting that job update can cause the pointer to be invalid
and thus should not be accessed after.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NPavel Mores <pmores@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>

src/qemu/qemu_blockjob.c

@@ -571,6 +571,7 @@ qemuBlockJobRefreshJobs(virQEMUDriverPtr driver,
 
         if (job->newstate != -1)
             qemuBlockJobUpdate(vm, job, QEMU_ASYNC_JOB_NONE);
+        /* 'job' may be invalid after this update */
     }
 
     /* remove data for job which qemu didn't report (the algorithm is