pool: Fix pf command regression

A recent patch changed a number of variables and fields from String to
FsPath. Inadvertently the return message of the 'pf' command in the pool
was changed to, leading to Serialization errors.

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/6713/