Avoid misleadingly computing a default value for the pool.conf path

It turns out that xapi.ml overwrites this with another default, stored
in xapi_globs.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Log a sensible error message if the pool.conf can't be read.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Remove assert()s which are supposed to trigger and are ignored; actually log when an error occurs.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Set the default pool.conf path to Fhs.etcdir / xcp / pool.conf

Signed-off-by: David Scott <dave.scott@eu.citrix.com>