v2v: -o rhev: Move inspection functions.

This is pure refactoring, no functional change.
v2v: Add comments to DOM module.
v2v: Remove Types.overlay ov_preallocation field.

The ov_preallocation field was essentially an implementation detail of
the V2v module.  There was no need to carry it around in the overlay
struct, and nothing else used it.  This also allows us to simplify
other code.
v2v: Remove the Types.overlay ov_source_format field.

Not used by anything and unlikely to be useful.
v2v: Inline the initialize_target function.

This function had got smaller and smaller until now we might as well
just inline it.  Having it after the main function just obscured the
order that the code runs.

No functional change, just refactoring.
v2v: Update TODO.