Replace ad-hoc config file handling with Xcp_service.configure

This is the same as other xapi daemons. It allows each option to
be read from a config file and overriden on the command-line.

Also, extend the default xapi.conf with comments describing all
available options which previously were undocumented.

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>
Signed-off-by: David Scott <dave.scott@eu.citrix.com>