storage: Ignore block devices that fail format detection

storage: Ignore block devices that fail format detection

https://bugzilla.redhat.com/show_bug.cgi?id=1276198

Prior to commit id '98322052' failure to saferead the block device would
cause an error to be logged and the device to be skipped while attempting
to discover/create a stable target path for a new LUN (NPIV).

This was because virStorageBackendSCSIFindLUs ignored errors from
processLU and virStorageBackendSCSINewLun.

Ignoring the failure allowed a multipath device with an "active" and
"ghost" to be present on the host with the "ghost" block device being
ignored. This patch will return a -2 to the caller indicating the desire
to ignore the block device since it cannot be used directly rather than
fail the pool startup.