Inventory: allow the command-line and config file to override the inventory path

Without this, all binaries end up with whatever path was hardcoded at
configure time. This allows (for example) an opam xenopsd to use the
same system configuration as an RPM xenopsd.

Fixes [xapi-project/xcp-idl#115]

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #34 from djs55/xcp-inventory

Allow the config file to override the inventory path