sysprep: Update comments.
sysprep: Replace flags list with side_effects object.

Previously callbacks would return a list of flags, such as []
or [`Created_files].

In this commit we introduce two new objects, filesystem_side_effects
and device_side_effects (the latter is not used yet).

The callbacks that create files now need to call

  side_effects#created_file ()

instead of returning flags.

There is no functional change in this patch.
builder: Add note about cloning to man page.
builder: Clearer note in man page about SELinux and where /.autorelabel file comes from.

It's not added by virt-builder.  It is present in the templates (added
by virt-sysprep in fact).
builder: Add tip for how to relabel SELinux guest at build time.

Thanks: purpleidea/James.