pool: Fix command string output for migration commands

Addresses the issue that 'migration move' and 'migration cache' commands
are shown as 'migration copy' commands in the output of 'migration ls',
'migration info' and when saved to the setup file.

Target: trunk
Request: 2.7
Request: 2.6
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6278/