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

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

Commit id 'bc444666f' 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: Jie Wang <wangjie88@huawei.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>