v2v: Simplify conversion dispatch code.

Refactoring, no functional change.
v2v: Change "Trimming the filesystems" message to something less scary.

Avoid copying unused and blank areas.  Avoid alarm and support calls too.
v2v: types.mli: Fix typo in comment.
v2v: Split Types.overlay into Types.target and Types.overlay structures.

Move the fields from Types.overlay which were related to the target
file out.  Now Types.overlay is only concerned with the actual qcow2
overlay files, not with a mixture of both files.

For every guest disk, there is one chain which looks like this:

   source_disk         overlay                 target
  +------------+      +----------------+      +---------------+
  | s_qemu_uri |<------ ov_source      |<------ target_overlay|
  | etc        |      | ov_overlay_file|      | target_file   |
  +------------+      | ov_sd          |      | target_format |
                      | ov_virtual_size|      +---------------+
                      +----------------+

   describes source    describes temp.         describes target
   disk file           overlay (qcow2)         file

This is just refactoring.