This patch fixes the problem reported in:

https://bugzilla.redhat.com/show_bug.cgi?id=509306

The bug reporter says that vol-delete does not support the --pool
option, but that's not the case in the current head.  This patch makes
vol-path behave the same way as vol-delete

* tools/virsh.c: Modified vol-path to use the same logic as vol-delete,
  allowing the syntax: virsh vol-path --pool testdirpool testvol0