blockjob: Fix a error checking of blockjob status in some case

Commit id 'bc444666' added a check if the returned data
buffer had an error, but failed to adjust the event from
VIR_DOMAIN_BLOCK_JOB_COMPLETED to VIR_DOMAIN_BLOCK_JOB_FAILED
in order to propagate an error such as "File descriptor in bad
state" that may be returned from QEMU when both @offset and
@len are set to 0 such as is the case when performing an async
block job read on a read only filesystem.
Signed-off-by: NJie Wang <wangjie88@huawei.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>

src/qemu/qemu_monitor_json.c

@@ -889,7 +889,7 @@ qemuMonitorJSONHandleBlockJobImpl(qemuMonitorPtr mon,
     case VIR_DOMAIN_BLOCK_JOB_COMPLETED:
         error = virJSONValueObjectGetString(data, "error");
         /* Make sure the whole device has been processed */
-        if (offset != len)
+        if (offset != len || error)
             event = VIR_DOMAIN_BLOCK_JOB_FAILED;
         break;
     case VIR_DOMAIN_BLOCK_JOB_CANCELED: