storage: use virDirRead API
    
    More instances of failure to report (unlikely) readdir errors.
    In one case, I chose to ignore them, given that a readdir error
    would be no different than timing out on the loop, where the
    fallback path behaves correctly either way.
    
    * src/storage/storage_backend.c (virStorageBackendStablePath):
    Ignore readdir errors.
    * src/storage/storage_backend_fs.c
    (virStorageBackendFileSystemRefresh): Report readdir errors.
    * src/storage/storage_backend_iscsi.c
    (virStorageBackendISCSIGetHostNumber): Likewise.
    * src/storage/storage_backend_scsi.c (getNewStyleBlockDevice)
    (getBlockDevice, virStorageBackendSCSIFindLUs): Likewise.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>