pnfsmanager: Fix compatibility with 2.6 pools

2.6 pools send PnfsGetStorageInfoMessage upon issuing the 'rh restore' command.
This message has been removed and thus pool compatibility with 2.6 is broken.
This patch adds a minimal version of that message back to ensure compatibility
with old pools.

Target: trunk
Request: 2.8
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6696/