xenserver: Don't create XAPI cache at install.

xenserver: Don't create XAPI cache at install.

There are two mechanisms of installing a supplemental pack – one is
after installation (using the xe-install-supplemental-pack command) and
one is during installation by either creating a new install ISO with the
supplemental pack bundled or creating an HTTP directory which serves the
install files and the new supplemental pack contents.

The RPM should support both methods, but currently attempts to backup
data that doesn't exist yet.  With this commit, the XAPI cache is not
created at install.

Signed-off-by: Bob Ball <bob.ball@citrix.com>
[removed sourcing /etc/xensource-inventory since it is no longer needed]
Signed-off-by: Justin Pettit <jpettit@nicira.com>