block/iscsi: only report an iSCSI Failure if we don't handle it gracefully

we currently report an "iSCSI Failure" in iscsi_co_generic_cb if the task
hasn't completed with SCSI_STATUS_GOOD. However, we expect a failure in
some cases and handle it gracefully. This is the case for misaligned UNMAPs
and WRITESAME10/16 calls without UNMAP. In this case a failure in the
logs can be quite misleading.

While we are at it improve the logging to reveal which operation failed
at what LBA.

Signed-off-by: Peter Lieven <pl@kamp.de>
Message-Id: <1512733868-9009-3-git-send-email-pl@kamp.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>