Add an 'errata' module to perform post-setup hacks

This is where we can put temporary workarounds provided
they are idempotent and provided that we plan to fix
the packages properly.

Famous last words?

The first errata runs "update-rc.d <service> defaults"
on Debian/Ubuntu systems since our packages fail to
run post-install actions.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
errata: on CentOS, make sure /usr/lib/xapi/sm points to the right place

The SM plugins are now being installed in /usr/lib64/xcp-sm.

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