storage: use virDirRead API

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>