scsi: Adjust return values from processLU

scsi: Adjust return values from processLU

https://bugzilla.redhat.com/show_bug.cgi?id=1171933

Adjust the processLU error returns to be a bit more logical. Currently,
the calling code cannot determine the difference between a non disk/lun
volume and a processed/found disk/lun. It can also not differentiate
between perhaps real/fatal error and one that won't necessarily stop
the code from finding other volumes.

After this patch virStorageBackendSCSIFindLUsInternal will stop processing
as soon as a "fatal" message occurs rather than continuting processing
for no apparent reason. It will also only set the *found value when
at least one of the processLU's was successful.

With the failed return, if the reason for the stop was that the pool
target path did not exist, was /dev, was /dev/, or did not start with
/dev, then iSCSI pool startup and refresh will fail.