util: Fix virStorageBackendSCSINewLun error handling

util: Fix virStorageBackendSCSINewLun error handling

Commit a523770c3 added @retval return processing for
virStorageBackendUpdateVolInfo in order to allow a -2
to be return; however, upon successful completion
@retval = 0 and if either the virStorageBackendSCSISerial
or the virStoragePoolObjAddVol failed, the method would
return 0, but not add the @vol to the pool. So let's
just reset retval = -1 and continue processing.

Signed-off-by: John Ferlan <jferlan@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>