Read the inventory file after the command-line and config file

The command-line and inventory file allow the inventory file path
to be configured, therefore we must delay reading it.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Add inventory and use-switch keys to the xapi.conf

The defaults from the code are written in comments, then
overridden with the XenServer defaults.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #1924 from djs55/fix-opam

Fixes to allow an opam binary to replace a dom0 one